The Departmental Secretary says he could not make a reasonable pay offer to his 19,000 public servants while the Abbott government was in power.

Departmental Secretary Dennis Richardson says it was only in late 2015, after Mr Abbott had been removed as prime minister and his public service Minister Eric Abetz was also sacked, that he could move forward with the enterprise bargaining process.

Mr Richardson said in his address that it was only after the bargaining policy had been softened in November 2015, in the wake of Mr Abbott and Mr Abetz's abrupt departures, that he felt he had been able to make a reasonable offer on a new pay and conditions deal.

As the Defence Department's civilian employees prepared to vote on Thursday for a third time on a new enterprise agreement, having already twice rejected proposals developed under the Coalition's hardline public sector bargaining policy, Mr Richardson has made an eight-minute video, appealing for his workers to vote "yes".

The secretary, who is expected to retire in 2017, said he held back from making an offer in 2014 and most of 2015 because he did not believe he could not make a reasonable proposal under the bargaining policy as it stood.

"I was particularly keen to hold back and move when I though there was a reasonable offer to put to staff," Mr Richardson said.

"It was in November of 2015, the government issued a revised bargaining policy and it was at that time I formed a view that we were in a position to make a reasonable offer."

Mr Richardson told his troops that he believed the arbitration process underway at the Immigration Department was not relevant to the dispute at Defence and that the Senate inquiry into the bargaining debacle was not likely to have an practical impact at Defence either.

"I believe that the more we hold back from an agreement, the more we'll fall behind those departments that have already voted their agreements," he said.

Mr Richardson's intervention came in the wake of a video plea by Consumer and Competition Commissioner Rod Simms, who urged his 800 public servants to put aside the unjust treatment they had endured and vote "yes" to a new proposal.

Technical union Professionals Australia says its members at a Defence base in Victoria were urged by a visiting senior departmental official to "gamble" on the new agreement in the hope their conditions and entitlements would be protected.

But the union PA, along with other workplace unions, is campaigning for a no-vote along with other workplace unions, telling its members that the new deal is still not fair.

"It's clear the Government didn't give Defence the capacity to provide what the secretary thinks was a fair offer until this year – and no compensation for the delay," union official Dave Smith said.

"It's not a surprise that the secretary has lost the appetite to push the agreement, senior executives selling the agreement around the country are openly describing the agreement as a gamble."