Russell John Tully, 48, has been charged with murdering Work and Income front-counter workers Peggy Turuhira Noble, 67, and Susan Leigh Cleveland, 55, on September 1 last year.
He is also accused of attempting to murder two other staff members - Lindy Louise Curtis, 43, who was shot in the leg and badly injured, and Kim Elizabeth Adams.
Tully faces charges of murder and attempted murder, and other charges that include theft, unlawful possession of firearms and ammunition, and setting a man trap.
Tully is remanded in custody for a further pre-trial call-over tomorrow.
The Public Service Association (PSA) said WorkSafe's move shows the importance that must be placed on health and safety in the workplace.
"WorkSafe have the statutory right to make independent decisions regarding prosecution and they won't have made this lightly," PSA national secretary Richard Wagstaff said.
"All relevant information and necessary resources should now be made available so that this matter can be resolved."
He said it was crucial that the state sector learned "every lesson possible" to ensure tragic events like the Ashburton shootings did not happen again.
"The PSA has worked alongside MSD through their own investigation and we will continue to assist them to make improvements in health and safety for all staff," Mr Wagstaff said.
"An increased focus on health and safety across the wider state sector is the most important change that must come from this tragedy.
"Everybody has the right to go to work knowing they'll be home safe at the end of the day."
In a statement this afternoon, Ministry of Social Development chief executive Brendan Boyle said he had been advised of the charge against the ministry.
"Once we know the full details of the charge, we will fully consider our position," he said.
"No further comment will be made as the matter is before the courts."
