Nothing more could have been done to stop the September 1 shootings of Ashburton Work and Income staff, the first stage of an independent review has found.
But the reviewers said the Ministry of Social Development (MSD) should do more to improve staff security generally.
Reviewers Murray Jack and Rob Robinson said "all practicable steps" were taken to ensure Work and Income staff safety and there was no way MSD could have prevented the unprecedented shootings.
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The reasons behind that finding have largely been withheld from the public, due to an ongoing police investigation and "privacy interests" but reviewers said MSD did what it could with the tools and policies it had at the time.
"It's extremely difficult to design a system to prevent someone who is determined to inflict extreme and violent harm," MSD chief executive Brendan Boyle said this afternoon.
"But we've made changes to enhance security for staff and the public."
Mr Boyle said that since the September 1 shootings, MSD had also adopted a "zero tolerance" approach to clients who made threats and was also seeking to change its organisational culture.
MSD had also introduced tighter controls on public access to its offices.
Homeless man Russell John Tully, 48, has been charged with murdering Work and Income front counter workers Peggy Noble, 67, and Leigh Cleveland, 55, on September 1.
He is also accused of attempting to murder two others. One of them was Lindy Curtis, 43, who was shot in the leg and badly injured.
The reviewers said that to forestall any future violent attacks, MSD should encourage more vigilant incident reporting and expand information-sharing with other government agencies -- and even non-government groups.
Mr Boyle said this potentially included agencies such as the Salvation Army and other social service providers.
Mr Boyle said that since the Ashburton shooting, Work and Income staff had received about 200 threats serious enough to consider reporting to police.
"There's definitely been an escalation since the first of September," he said. These included verbal threats and threats to "do an Ashburton" on other Work and Income staff.
MSD said it had hired 200 more security guards to protect staff around the country since the Ashburton shooting.
Mr Boyle said it was now apparent Work and Income employees had tolerated too much bad behaviour in the past.
He said it was currently not an option to recommend cutting entitlements to threatening or abusive clients, as this was a complex policy issue.
The report marks the first phase of a review into the shooting. Further enquiries will look at all ministry workplaces and will recommend any further measures that might be needed to improve public and staff safety.