The Ministry of Social Development admits breaching health and safety legislation over gunman Russell John Tully's killing spree.

But it disputes some allegations against it.

The double-murderer, who is serving at least 27 years in prison, stormed Ashburton's Work and Income office on September 1, 2014.

From pointblank range he shot dead receptionist Peggy Noble, 67, and Susan Leigh Cleveland, 55, as the case manager begged for her life.

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Tully, 50, was also earlier this year found guilty of attempting to murder case manager Lindy Curtis.

In the Wellington District Court today, the ministry pleaded guilty to breaching the Health and Safety Act, a prosecution brought by WorkSafe NZ.

There will now be a hearing about disputed facts.

The disputed aspects of the case are to do with the layout or "architectural security" if the office.

Ministry lawyer Brett Stanaway also asked for no conviction to be entered in case the ministry were to apply for a discharge without conviction.

In a statement, ministry chief Brendan Boyle said the guilty plea was a "pragmatic step".

"Proceeding with defending the charge as a whole would be a pointless exercise given the courts had already determined that Russell Tully was solely responsible for the murders."

Mr Boyle said if he thought going to a trial would provide more insights into what happened he would do so.

"Former police commissioner Rob Robinson and Deloitte managing partner Murray Jack, backed by a senior advisory team that included representatives from Australia's Centrelink and the ANZ bank, found nothing could have stopped a motivated murderer like Mr Tully.

"MSD is disputing the aspect of WorkSafe's prosecution relating to its interpretation of what a safe office layout should be. I do not accept that organisations working with the public need to do that from behind barriers.

"For MSD, and for the public service as a whole, interacting with clients in an atmosphere of relative openness is a key to successfully working with them. WorkSafe believe a more rigid and less welcoming approach is required even though the Ashburton attack was at the most extreme end of risk for any agency that deals with people - public or private.

"There are potential far reaching consequences for all entities that deal with the public, if WorkSafe's perspective on this matter is allowed to stand," Mr Boyle said.

Mr Boyle said WorkSafe had indicated "some practicable steps MSD needed to take to ensure a safe working environment for all staff".

"Our plea recognises that to some degree.

"We had been steadily implementing many of these steps before the Ashburton tragedy, and we increased the pace of implementation following it. These steps had no causal link to Mr Tully's crimes. The security of all MSD staff is a top priority for the organisation," Mr Boyle said.

"We would never knowingly put our staff at risk. Ashburton was extraordinary in that Mr Tully was motivated to kill. Nothing can help someone intent on mayhem."

Today's plea would help bring everyone at the ministry a step close to putting the Ashburton tragedy behind them, Mr Boyle said.

Mr Stanaway said the court didn't need to find it guilty of the "practicable step" to do with layout, given the ministry pleaded guilty to breaching five other steps and it would make no difference at sentencing.

He said a hearing about disputed facts, potentially lasting three to five days, was not required.

"The attack, by all accounts, took less than a minute in total. All the victims were known to Mr Tully, they having interactions with him over a period of time," Mr Stanaway said.

The attack was planned and "ferocious".

"Mr Tully was clearly identifying certain persons within the office, knew who they were and what they looked like, and could have had access to them outside the office," Mr Stanaway said.

"It is the defendant's case that the attack was irresistible."

Since the tragedy, the ministry had made ex gratia payments to those affected, among other steps taken to make amends.

The levels of payment, suppressed for the moment, were about what any reparation order would be, Mr Stanaway said.

The ministry had been a good employer, providing financial and "other assistance" to victims "above and beyond what would normally be expected from an employer in this context".

In this matter, the charge focused on the Ashburton office, but WorkSafe appeared to be using this as a test case about "architectural security" nationwide.

After the shooting the Ashburton office was cordoned off as a crime scene. Work and Income stopped its lease and the building is now used for something else.

"That is a defunct office. It no longer exists and we know that the architectural security of that office is no longer current," Mr Stanaway said.

"The risk climate that existed prior to September 1, 2014, is materially different to what exists after that event. It was, in my submission, an exceptional event that could not have been reasonably anticipated in a small market town in rural New Zealand."

Assessing the Ashburton office as it was in 2014 was an historic exercise with little current value.

Mr Stanaway said the ministry could not be accused of lacking accountability and transparency.

A wide-ranging independent report into what happened and security issues was publicly available and the ministry had since worked "assiduously" to ensure lessons learned from the tragedy were acted on.

WorkSafe prosecutor Dale La Hood said the issue of layout was important.

While the ex gratia payments were at a level expected, that was only one aspect of sentencing, the most important of which was court denunciation.

It was "quite unusual" to suggest the independent report into the tragedy covered all points relevant to this prosecution, Mr La Hood said.

Chief District Court judge Jan-Marie Doogue took the court through the details of the charge the ministry admitted - that it failed to take all practicable steps to ensure staff were not exposed to hostile or violent clients.

The ministry is disputing that it failed to ensure there was no physical access to staff by clients.

Judge Doogue declined the ministry application to not hold a full hearing into disputed facts.

She said if the ministry was found to have breached the part of the charge it disputed, it would have an effect on sentencing.