Public service unions are targeting the lack of backpay in the federal government's new generation of wage offers, arguing the deals are worth much less than departments claim.
The CPSU says the offer of 6 per cent, over three years, to 18,000 Defence public servants is "a fiction" because their pay has been effectively frozen by the bargaining stalemate since 2013.
The latest ballot development in the saga, an emphatic rejection of a wage offer by public servants in Malcolm Turnbull's Department of the Prime Minister and Cabinet, brought an attack by the opposition on the entire bargaining policy.
Seventy four per cent of the department's 2400 public servants voted with 68 per cent voting no.
The news follows no-votes at the Electoral Commission, National Archives and the Australian War Memorial, while the Employment Department voted yes and the Australian Sports Commission returned a landslide approval for its new deal.
With a massive workplace ballot underway at Defence, departmental secretary Dennis Richardson tackled the backpay issue head-on last week, telling his workers that wages had never been backdated in any agency or department, to his knowledge.
But another workplace union, Professionals Australia, have found a precedent with back pay included in a Defence deal agreed in 2002 and have asked Mr Richardson to correct the record.
The back pay ban, a cornerstone of the Coalition government's tough public service workplace bargaining policy, is emerging as an issue in the disputes because most agreements in the public service expired in mid-2014 and there have been few pay rises since 2013.
CPSU has hit the backpay issue hard as it campaigns for a no-vote in Defence.
"Don't be fooled by the management fiction that your pay offer is 6 per cent over three years," a union bulletin to members said.
"When the pay quantum is properly calculated from the nominal expiry date of the current agreement it equates to below 1.3 per cent per annum."
But the Public Service Commission is dismissive of the unions' arguments, saying the backpay ban simply follows a "long held principle".
"The Workplace Bargaining Policy 2015 continues a long held principle that remuneration increases are to be prospective," a commission spokesperson said on Monday.
"This is a principle that has existed in successive bargaining policies and frameworks for many years.
"The way for employees to gain a pay increase is to vote yes when new agreements are put forward."
Labor's employment spokesman Brendan O'Connor seized on the PM&C vote on Monday to call for a rethink of the public sector bargaining policy.
"With yet another no-vote, it is clear the Government's workplace bargaining policy remains unfair, unworkable, and ideological – it should be abandoned immediately," Mr O'Connor said.
"Sixty-eight per cent of the department staff voted'no' to the proposed enterprise agreement, for the second time.
"The Abbott-Turnbull Government has reduced collective bargaining to an adversarial zero sum game epitomised by confrontation and ideology.
"Mr Turnbull's aggressive anti-worker industrial agenda will not assist the public service to meet increasing demands on government service delivery and it does not recognise the contribution of public servants who perform complex and demanding tasks on a daily basis."