Mark Stuart Wallis, who is now a South Coast school bus driver, pleaded guilty in the ACT Supreme Court on Thursday to three charges.

These were two counts of dishonestly causing a loss to a Commonwealth entity, and a single charge of intentionally using a forged document to obtain a gain.

Agreed facts show the former Commonwealth Scientific and Industrial Research Organisation director of business and infrastructure committed the offences between July 2012 and June 2017.

The 47-year-old's crimes included making nearly $72,000 worth of personal purchases with a corporate credit card.

One "particularly egregious" purchase was that of a jet ski, for which Wallis splashed out nearly $26,000. He later "demonstrated a high degree of dishonesty", as a prosecutor put it, by claiming the payment related to repairs for CSIRO boats.

Wallis, who once managed a team of more than 300 people, also invoiced the CSIRO for gym memberships and personal training services for himself and his wife.

While the scientific research agency had agreed to pay for his as part of "an early intervention process to deal with stress and other issues", Wallis was not authorised to use government funds for the $5275 he caused the CSIRO to spend on his wife's fitness.

Finally, Wallis used government money to obtain for himself a Lexus station wagon and a Toyota Hilux worth a combined amount of more than $245,000.

He did this by doctoring a Canberra Toyota invoice and having a subordinate process it, disguising the purchases as fees for the termination of leases for 10 CSIRO fleet vehicles.

After the CSIRO became aware in mid-2017 of Wallis' misdeeds, he told the agency's human resources director words to the effect of: "I don't know how this happened. It's serious."

However, the following day, he conceded the purchases were "100 per cent mine".

"I can pay CSIRO back," he said. "Just take it out of the money you owe me."

Following that admission, in July 2017, the CSIRO sacked Wallis for misconduct and federal police raided the Griffith home he occupied at the time.

Wallis ended up repaying the agency the defrauded amount in October 2018.

Criminal proceedings against Wallis, who also once worked for the Australian Federal Police, were initiated the following month.

In written submissions tendered to the court on Thursday, federal prosecutor Natasha Purvis described Wallis' offending as "calculated, persistent and manipulative".

Ms Purvis also said the crimes represented a significant breach of trust, and that they were so serious "a term of immediate imprisonment is required".

The prosecutor added that a pre-sentence report suggested Wallis had shown "limited remorse and contrition".

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That report says Wallis, a father of three whose marriage broke down because of his crimes, took full responsibility for his offending but denied having any memory of carrying it out.

"I have lost everything due to this," the "highly emotional" offender told the report's author.

Wallis' brother, Baptist minister Daniel Wallis, gave evidence on Thursday that the offender had changed, becoming forgetful and more irritable, in the wake of a bad motorcycle crash in 2010.

"Before the accident, he was on top of the world," Reverend Wallis told the court.

Reverend Wallis added that the offender began complaining to him in about 2014 that his boss at the CSIRO was bullying him, crying "uncontrollably" on the phone about this.

In her oral submissions, Ms Purvis said this was not a mitigating factor.

"A very clear message has to be sent to the community that if people are facing difficulties in a workplace, they cannot resort to ... extreme fraudulent behaviour".

Defence barrister Margaret Jones SC, instructed by solicitor Michael Kukulies-Smith, urged Chief Justice Lucy McCallum to impose a fully suspended jail sentence.

Ms Jones referred to the alleged bullying and stressed that "complex mental health issues" were at play during the offending, which had "no apparent or logical" explanation.

She also reiterated her client's comment that he had "lost everything" as a result of his crimes.

Wallis, who has been assessed as not suitable for an intensive correction order, faces a jail sentence of up to 20 years.

His sentence hearing is due to continue on May 6.

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