Updated 9.12am: Anne Tolley is saying little about a charge against the Ministry of Social Development, over last year's Ashburton Work and Income shooting.

WorkSafe NZ has charged the Ministry with failing to take all practicable steps to ensure the safety of its employees at work in the September tragedy.

The Social Development Minister won't comment at any length until the case has run its course.

But she says it's important to learn about what happened, and what could have been done to prevent the killings.

PSA national secretary Richard Wagstaff says it's important lessons are learnt.

"Many many departments have interactions with the public, and many of those interactions are difficult. There will be a lot of people watching this case."

Richard Wagstaff says in taking another government department to court, Worksafe would have realised the headlines this action would make.

"From our point of view if it's going to make people look harder at what could be done better, that's not a bad thing. It certainly is a wake up call."

Peggy Noble and Leigh Cleveland were both killed in the tragedy.

Their alleged killer, Russell Tully, is also charged with the attempted murder of two others - but has yet to enter a plea.