National campaigned on slashing "back-office expenditure" across 24 public agencies, as part of its "Back Pocket Boost" tax plan.
However, Finance Minister Nicola Willis has confirmed the government has widened the scope, and has now asked "all departments" to identify savings.
Public Service Association assistant secretary Fleur Fitzsimons told Midday Report this was "a very short-sighted approach".
"It will also undermine the ability of the government to implement evidence-based policies.
"They say they want to take an evidence-based approach. Well, I thinking cutting the public service is not a great way to start."
New Zealand needed public sector expertise to deal with a range of looming problems, including the infrastructure deficit, the ageing population and the need to reduce emissions, she said.
However, it appeared that public servants were being "sacrificed" to pay for the promised tax cuts with all agencies asked to find "cost savings" of at least 6.5 percent.
"That's particularly worrying as many of those agencies and research institutes are already cut to the bone and operating on the smell of an oily rag."
Union officials had met with the Finance Minister and made their views clear that these cuts were not only unnecessary but would have a damaging effect, Fitzsimons said.
Meanwhile, the PSA was continuing to work with its members and government agencies to see where efficiencies could be made without job losses.
"But the reality is the savings that have been asked for are going to cut deep and we're going to be monitoring any job losses and the impact on services."
Contrary to claims of a blow-out in the number of state employees, the growth of the public sector in the last few years was in line with population growth and comparable to that of other countries, she added.
RNZ
