Defence Minister Judith Collins has used special powers to allow soldiers to carry out civilian work as Defence Force staff strike over pay.
About 1000 civilian workers have ramped up industrial action in the last week, with NZDF refusing to offer a pay increase.
Staff have been working to rule, refusing overtime, taking combined breaks and stopping any work at heights or off-site.
Under the Defence Act, uniformed personnel can only be used during an industrial dispute if the minister of defence authorises it in writing - which Collins confirmed she had done.
"While industrial action is ongoing the essential functions of security and firefighting need to be covered by uniformed personnel," she said.
"I have accordingly authorised the use of uniformed personnel to cover duties undertaken by the Defence Security Guard Force and NZDF airfield firefighters, which are critical risk areas and roles, on the grounds of safety or health, protecting national security and maintaining emergency management requirements."
Public Service Association (PSA) assistant secretary Fleur Fitzsimons said the "extraordinary" move, which had not been done since 2001, was an attempt to undermine the strike.
But replacing the staff showed how important and valuable they were, she said.
"The military clearly could not operate without civilian personnel, which is why the zero-pay offer is insulting.
"NZDF urgently needs to start valuing civilian workers just like those in uniform."
Civilian staff wanted a pay rise that reflected the increased cost of living and the importance of their work, Fitzsimons said.
The PSA was seeking urgent legal advice on whether Collins' move was lawful, given the strict provisions in the Employment Relations Act about when and how employers can replace striking workers, she said.
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