Dumped defence contractor Serco will have to pay hundreds of thousands in redundancies to up to 200 Canberra workers sacked after the Britain-based multinational lost the facilities contract for defence sites around the capital.

The Fair Work Commission has Serco, and its joint venture partner Sodexo, pay the cleaners, cooks and security guards, many of whom worked at the Royal Military College Duntroon, after a dispute over which workers were owed severance pay-outs.

The services union United Voice said the FWC decision had exposed the behaviour of the joint venture partners towards its former workers as "ugly and mean".

The union says the workers, many of whom earned about $550 a week, might get between $4000 and $5000 each.

The joint venture Serco Sodexo Defence Services (SSDS) employed about 1400 workers in the ACT and parts of surrounding NSW, until it failed in a tender for Defence Department work last year.

SSDS supplied catering security and cleaning services to the military college and other military and defence installations, employing about 300 people, including about 100 casual employees.

When the joint venture failed in its bid to have its contract renewed in 2014, the workers were sacked with many of them finding work with the companies who took over the contracts.

Under Fair Work laws, a company does not have to pay a redundancy to a worker if the employer has secured a new job, with similar pay and conditions, for its former employee.

SSDS refused to pay redundancies to many of its Canberra workers, arguing that it had helped to get them jobs with the new contractors at defence, freeing the joint venture from the legal obligation to pay-out their former employers.

But after a three-day hearing in Sydney in November, Fair Work Commissioner Julius Roe found, in a decision handed down on Thursday, the joint venture had not played as decisive a role in its former employees' new jobs as it claimed.

Workers gave evidence that they had heard about the jobs with the new contractors from a variety of sources, including Defence Department public servants.

"I am satisfied that SSDS was not a strong moving force behind the SSDS employees being offered employment with Transfield, Wilson, Brookfield, Menzies, AFL, Blackhawk, Spotless or Compass," commissioner Roe wrote in his decision

"The actions of SSDS were insufficient to cause acceptable alternative employment to become available to the redundant employees."

But the company still has a chance to reduce its liability with commissioner Roe agreeing to hear more legal arguments on the amounts to be paid.

A spokesperson from SSDS would not comment other than saying the company had noted the decision and was considering its position.

Tony Cabello, a security guard at the Defence Department's Russell Offices, said he and his colleagues had been shocked when they lost their jobs at SSDS and had been given little help in securing new employment.

"Some of us were lucky to get employed but it wasn't a very nice Christmas," the father-of-four said.

United Voice ACT Branch Secretary Lyndal Ryan was scathing of the joint venture partners' behaviour towards its former workers.

"This is the ugly and mean side of giant corporations," Ms Ryan said,

"They can't be allowed to flout local laws to rob hard-working loyal staff of their entitlements.

"This money is a drop in the ocean for SSDS but is a huge amount of money to those workers who have been left waiting."