The 58-year-old former barrister was fuming over what he saw as an attempt by the government-owned shipbuilder, the Australian Submarine Corporation, to sabotage him several days earlier. During a Senate inquiry, Stuart Whiley, the acting head of ASC, had appeared to lowball the price it would cost to build 12 new submarines locally – citing a figure of between $18 billion and $24 billion, roughly half what the government believed it would cost.

Johnston saw this as a direct attempt to undermine the government's increasing attraction to sourcing the submarines more cheaply offshore, perhaps even from Japan, given the new chumminess between that country's prime minister Shinzo Abe and Tony Abbott. And it came on top of his mounting frustration over the troubled Air Warfare Destroyer program, in which ASC plays a key role.

Seeing an opportunity on Tuesday to create mischief, Labor senators homed in with a few well-aimed questions. They struck political gold.

Johnston began paying out on the ASC in spades, suggesting it had no more than a one-page document to justify its submarine costings and appearing to mock its staff by suggesting they were just looking for jobs. Goaded by jibes from Labor, he went further, eventually bellowing across the chamber, "you wonder why I wouldn't trust them to build a canoe?"

It was a reckless comment from the man nominally in charge of the process. Johnston didn't stop to think what effect his remarks might have on the morale of ASC's staff, or on the navy's submariners, who entrust their lives to the maintenance the ASC performs on Australia's existing Collins class submarines.

The next morning, facing  a rare censure motion by the Senate, Johnston tried to dismiss his outburst as a regrettable "rhetorical flourish". Yet  even his supporters were admitting the remarks were plain dumb. South Australian MPs, who face the wrath of voters if the Adelaide-based submarine program and its thousands of jobs are sent offshore, were furious.

More seriously for the government, it brought to the surface long-simmering issues over Johnston's competence, rivalries within Cabinet and the centralisation of power in the Prime Minister's office.

Tony Abbott, who went to last year's election vowing to scrap the carbon tax, stop the boats and balance the budget, has had Defence thrust to the top of his list by international events such as the rise of Islamic State and the downing of flight MH17, and domestic controversies over defence pay, and fresh revelations this week about the scale of sexual and physical abuse inside the defence force.

Those are just the immediate issues. Asia is going through a profound strategic shift to which Australia's policy makers are still trying to formulate a calibrated response.

The canoe outburst thrust into the open a question many were asking privately: was David Johnston up to the job?

The weight of opinion is that Abbott will soon decide he is not, with many commentators tipping Johnston would be among the first moved in any Cabinet reshuffle. That said, Abbott firmly backed Johnston this week, and government insiders point to the Prime Minister's track record of making only rare changes to his team.

Still, unfavourable comparisons are being drawn with Immigration minister Scott Morrison's record. Love or loathe his policies, Morrison has delivered beyond expectations on what Abbott asked him to do - and he is said to want the Defence job.

Johnston, by contrast, has been seen as  ineffectual and gaffe-prone. And this week he notably failed to front for a press conference or interview on two landmark reports brought down on sexual assault and abuse in the military, instead making only a brief statement to the Senate.

Former and current defence insiders describe Johnston as a "nice guy" who is strong on technical detail. He can talk anyone under the table on the intricacies of submarine propulsion systems or the body armour systems worn by Special Forces soldiers.

But he is seen as weak on the bigger strategic picture, and a poor prosecutor of a political case, including to his Cabinet colleagues, and as too easily steamrolled by powerful figures in the Prime Minister's office.

"He can talk nuts and bolts and serial numbers, but he has never worked out how it all fits together strategically," says former deputy of the Defence department Hugh White, echoing a view shared by a number of others who would only speak off the record.

Allan Behm, a former senior defence official who worked for Greg Combet when Combet was defence materiel minister, agrees.

"The problem with Senator Johnston is that he doesn't really understand the dynamics and complexity of the task that Defence faces, both in fielding a fighting force virtually at a moment's notice, and at the same time undertaking what are very complex and very long-term procurements," Behm says.

Johnston, he says, "is a very decent man, but he is simply not a breakthrough type person and he is not decisive".

Another source who has observed Johnston's style  says: "Almost all the problems that David faces are his fault. He is a talker, not a doer - too often he agrees with the last person he spoke to. In defence, you've got to be the boss."

The big picture stuff is instead being handled by the Prime Minister and his office, particularly by the intellectual driving force on international security, former diplomat Andrew Shearer. Or on the strategic side, it is Foreign minister Julie Bishop who does most of the talking. As it happens, she is also Johnston's most powerful backer – some say his lifeline as a minister - their close alliance dating far back in Western Australian Liberal politics.

"He is desperately overshadowed by Julie Bishop," Behm says. "If you saw him in Washington or at the NATO meetings, Julie Bishop does all the talking but these are essentially defence meetings."

Journalists calling Johnston's office to ask about Iraq are routinely referred to Abbott's office.

It's not unique for the Prime Minister to take the lead on defence at a time of international conflict. Howard's defence minister Robert Hill during the last Iraq War used to refer to himself acidly as "the minister assisting the prime minister for defence".

But insiders say it is amplified in Johnston's case because his standing with the PM's office is marginal.

"I don't ever remember a government that has operated this level of control over a senior minister," one security veteran said.

Does this actually matter? Supporters of Johnston points out that it makes sense for the defence minister to concentrate on the nuts and bolts stuff, that his detailed technical knowledge allows him to hold Defence to account and make a useful contribution to meetings of the National Security Committee of Cabinet.

But there have been some odd ideas floated. Abbott originally wanted to send about 1000 Australian soldiers to Ukraine to help secure the crash site of MH17, but was told such a force on Russia's doorstep was dangerous. He is also keen on a jump-jet variant of the Joint Strike Fighter, turning the new amphibious warships – one of which, the HMAS Canberra was commissioned in Sydney on Friday – into aircraft carriers. It is widely seen as unfeasible.

To be sure, the wheels haven't fallen off defence. But the minister's weakness means things aren't going as well as they could, some observers say.

"You can get away with it when nothing much is going on," one well-placed source says. "But this is the busiest national security committee … that we have laid eyes on for a long, long time. They have a lot on their plate, so if you are trying to get to Andrew Shearer or to Abbott , you stand in a very, very long line and, as a result, not much happens. There is no intellectual time left in a lot of these people's heads."

Allan Behm puts it this way: "The best counsel for Abbott would be to find a more energetic and confident defence minister."

It's also been a rocky road inside Johnston's own office. Along with several other ministers, he had his first chief of staff, senior defence official Simeon Gilding, imposed on him by Abbott's office. Johnston chafed at the imposition, clashing with Gilding and eventually demanding a new chief of staff – a move his supporters say shows he can stand up for himself.

Yet Johnston was blocked when he tried to take the task of steering the forthcoming 2015 Defence White paper out of the hands of his own department and give it to prominent defence analyst Alan Dupont. He also chose former defence official Ross Babbage for another key advisory role. But both appointments were overridden in what one observer termed a pincer movement by Abbott's's adviser Shearer and Defence department head Dennis Richardson.

"Johnston was unable to prevent that happening, even though he had personally invested his authority in both appointments," one first-hand observer said.

But the stoush that really spilled over into the public sphere came after former general Jim Molan - who helped draw up Abbott's boats strategy - went to work for Johnston in August. He clashed with Johnston's new chief of staff, Sean Costello, and the arrangement lasted just three weeks.

Molan told Fairfax Media that he encountered interference as soon as he took up the job. The first time he visited the office and asked to speak with Johnston, he was told to make an appointment, he says.

Supporters point out that Johnston has had successes: cracking the whip on the beleaguered Air Warfare Destroyer project, and winning a six per cent funding increase this year while other portfolios were slashed.

Other sympathisers point out that its an incredibly hard job, given the size, complexity and peculiar internal culture of Defence. White calls it a "minister killer".

If there is one achievement that would secure a legacy for Johnston, it is solving the submarine conundrum.  But to steer a course through the high passions, vested interests, political pitfalls and sheer intimidating gravity of the submarine decision requires a strong, focused minister.

Johnston, who speaks some Japanese, was interested in the country's Soryu Class boats even while in opposition – well before Abbott and counterpart Shinzo Abe hit it off as leaders and set the stage for a sharing of submarine technology.

One seasoned observer says Johnston was too eager to go down that path.  "He got into a pickle by appearing to be less than principled and dispassionate. He vindicated his preference (for Japan) before they had done any proper analysis."

Now the field seems open again – largely a three way race among Japan, Germany and France with Sweden a distant fourth.

"The world must look at us and shake their head," says Mark Thomson of the Australian Strategic Policy Institute. "I talk to the shipbuilders from overseas and the people who want to sell us submarines and they're frustrated because they don't know what is going on."

The right decision on submarines would give Johnston a place in history. But if the rife speculation is correct and he has just months left, that chance has already slipped through his fingers.