But a Canberra court must decide how much of the payout the three defendants – an ACT government authority, his partner Constable Michael Hannaford, and the state of NSW – should cover.

Serial car thief Justin "Mully" Williams was drunk and behind the wheel of a stolen car when he led Edward Jausnik and partner Constable Michael Hannaford on a high speed-pursuit from Queanbeyan into the ACT on March 20, 2010.

The police called off the pursuit shortly before Williams ran an Ipswich Street red light and slammed into a car killing a young family.

The impact of the crash – which cut the family's car in half – killed Scott Oppelaar, his partner Samantha Ford, and their three-month-old son, Brody.

Williams died in hospital that same night, while his passenger, teenager Skye Webbe, suffered serious injuries.

Mr Jausnik said he had been forced to perform first aid on the victims and now suffers post-traumatic stress disorder and depression as a result.

Coroner Peter Dingwall found, at inquest, Williams responsible, describing his actions as "unlawful, reckless and selfish behaviour of a person who placed his desire to evade the police ... ahead of the lives of other road users".

In 2012, an ACT Supreme Court case brought by Mr Jausnik alleged Williams' negligence caused the accident, which would make the Office of the Nominal Defendant of the ACT liable, as it exists to compensate those injured by uninsured drivers.

The claim was settled by a consent judgment in July 2013, on the basis Mr Jausnik be paid $1.225 million.

The Office of the Nominal Defendant of the ACT then joined Mr Hannaford and the state of NSW as third parties to recover some of the funds, arguing they should contribute to Mr Jausnik's payout.

The ACT statutory office alleges Mr Hannaford continued the pursuit when he should have broken the chase.

NSW was alleged to have failed to provide training or instruction to officers concerning the pursuit of vehicles, including the termination of pursuits, and failed to provide training in the use of radio equipment to obtain instructions on the pursuit.

Both deny the allegations, and Mr Hannaford also claims Mr Jausnik contributed to his own negligence.

Much of the case was heard, including evidence, during a week-long hearing before Associate Justice David Mossop in May.

The matter was scheduled to return next month for a further two days of submissions.

But Mr Hannaford applied for leave to amend his defence to include a section of the Police Act that says officers are not liable for injury caused while acting in good faith on the job.

If allowed, the provision could provide a complete defence for Mr Hannaford.

The Nominal Defendant opposed the bid, saying its case would be prejudiced by the change and would require the entire case to be relitigated.

It would also mean its case against NSW ran into limitation issues.

Lawyers for Mr Hannaford said an oversight meant the section of the act had not been included as part of the original defence.

The lawyers said the court was obliged to apply the law and the provision was in terms that required it to be applied.

Acting Justice Mossop, in a decision published earlier this month, refused the application as it had been out-of-time.