Corrections has sought approval to expand the 1250-bed prison to 3000 beds, but the Government has repeatedly put off a decision on it.
Public Service Association organiser Willie Cochrane urged Corrections Minister Kelvin Davis to make a definitive statement of what the Government's plans were for Waikeria.
"If that expansion isn't going ahead, we want to hear what more he'll do to expand the capacity of our prisons in the short term and keep our members safe in the workplace," he said.
Morale among corrections officers was at an all-time low, he said.
Prisons are so overcrowded that staff were unable to provide rehabilitation to inmates and stress levels were "through the roof".
That has had a knock-on effect on probation officers, who are dealing not only with more prisoners but more complex cases as a result of the difficulties in providing rehabilitation.
The Budget includes a boost to probation services - $82m will be spent over four years, partly to provide 270 more probation officers. There are currently around 1200 probation officers.
Last year's Budget also increased the probation service by 100 officers, though unions said those increases had not kept pace with the growing prison population.
The prison muster is currently 10,650, leaving just 350 spare beds. It is forecast to rise to 12,000 by 2025.
Funding has already been allocated to expand Waikeria but the Government is still deciding whether to approve the full Corrections proposal, or opt for a more moderate expansion.
It is in a difficult position because it has committed to cutting the prison population by 30 per cent in 15 years.
Changes to bail laws are one of the options being considered to reduce the muster. The PSA warned that such a move would require hundreds more probation officers.
Cochrane applauded the target to reduce the prison population, but said decisive action needed to be made in the short-term.
Davis has been asked for comment.
NZ First MP Shane Jones said he was not involved in the Waikeria decision, but he was horrified by the cost projections for Corrections and supported Justice Minister Andrew Little's review of policy in a bid to reduce the prison population.
"What the hell are we going to do about the spiralling costs of running our penal system?
"If you think the cost of welfare, superannuation, health and education is growing exorbitantly have a look at this bloody Budget and have a look at how much dough we are spending on Corrections."
"I've got to tell you as a Maori, as a politician, as a grandfather and father I see very dangerous signs there as to what in the medium to long term, what sort of society we are creating.
"And for that reason I totally support Andrew Little wanting to have a hard long look at the justification for continuing on with the mega prison on the Waikeria site."
He said dealing with the cause of that offending would require "much carrot and stick" and getting the state to work more innovatively with young people.