Military personnel have had their Christmas and New Year leave cancelled to fill gaps left by strikes, says a lobby group.
Defence Minister Judith Collins last month invoked a rarely-used power to allow uniformed service men and women to fill in for striking civilian staff.
But the Mission Homefront group has written to her, saying this was unprecedented and unacceptable.
"The orders had led to Christmas and New Year leave being cancelled for many in uniform," said the group this week.
"More time away from their families adds to existing stress and contributes to a decrease in morale for people who are already managing unrealistic workloads and working multiple roles.
"Being forced to fill in civilian tasks such as gate security, performed day and night, is stretching people's patience."
They were already struggling with their own tasks, lack of training, low pay and inadequate welfare support, the letter said.
The use of the temporary power undermined the crucial interdependency with civilian staff, it added.
"Some civilians who are striking say it's incredibly difficult to watch uniformed staff being called in to do civilian jobs, knowing they are already overstretched.
"The sentiment among uniformed personnel with the section nine measure is overwhelmingly one of frustration, feeling the situation is disrespectful to both themselves and their civilian counterparts."
Getting uniformed staff to backfill for civilians was symptomatic of historical chronic underfunding of defence, the group added.
The NZDF said about 50 people across all ranks were being rostered on to fill gaps for camp and base security guards through to the end of January. This had allowed personnel to plan and arrange leave to maximise time spent with loved ones during this period.
It said it was bargaining in good faith with the PSA.
Collins has written back to the Mission Homefront group, telling them she had been advised by the Defence Force that it had contingency plans in place to minimise the impact of increasing industrial action.
She said the government was grateful to all staff who were working over the holiday period.
The PSA union said it and NZDF had jointly agreed to ask the Employment Relations Authority to provide facilitation and were optimistic it would.
"This dispute can be settled earlier if NZDF come to the bargaining table with a fair pay offer, we encourage them to do so," the PSA said.
The Chief of Defence recently told parliament that training, travel and building relationships with overseas partners were among the activities that would have to be paused or stopped in the face of a mounting deficit that hit $160m last year, and was heading for $360m next year.
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