Most EPMU delegates voted for Mr Cunliffe in last year's leadership runoff - in which strong support from the party membership and unions carried him over the line. However, the union's general secretary, Bill Newson, said that did not mean Mr Cunliffe would get that support again.
"I've seen comments that the party membership and unions will line up with David Cunliffe. People shouldn't make assumptions based on what happened last time. Because quite a big thing happened in the meantime and it was called an election."
He said the union would discuss its stance once the picture was clearer, and it was still possible delegates would support Mr Cunliffe if there was a contest.
It had not made a formal recommendation to its members last time, and only about 35 of its 80 delegates had voted.
Mr Cunliffe is expected to announce his intentions today after a meeting of Labour's ruling council. He is expected to resign but could delay a decision on whether to challenge for the role again until after a review of the election is finished in December.
Mr Cunliffe is understood to be convinced he could win another party-wide contest - but the EPMU's response indicates some of those unions are rethinking their position.
Other unions refused to comment and only the Meat Workers' Union said it remained behind Mr Cunliffe.
National secretary Graham Cooke said the feedback from union members was strongly pro-Cunliffe.
"They think he's been given a very raw deal by the right-wing bloggers, Cameron Slater and commentators like Paul Henry and Mike Hosking. Everyone's just been gunning for him."
He said it was up to the party to decide if there was a contest, but if Mr Cunliffe was a contender he would vote for him.
The union vote counts for 20 per cent of the final tally in the leadership contest. The EPMU's size means its delegates' vote is worth 35 per cent of that. The members' and caucus votes are worth 40 per cent each.
Mr Cunliffe has spent the four days since Tuesday's bruising caucus meeting holed up at his Herne Bay home talking to advisers and popping out for an occasional stroll on the beach, where he was spotted by a Weekend Herald photographer yesterday. Some are understood to have counselled him to stand down and not re-contest the post even if he believes he can win it.
Supporters of another likely candidate, Grant Robertson, suspect Mr Cunliffe's followers of leaking a breakdown of last year's Labour leadership election results, which showed just how far Mr Robertson was behind Mr Cunliffe in the union and membership votes.
Robertson supporters suspected the leak was to discredit Mr Robertson's claim for the job.
Since Tuesday's caucus meeting, Mr Robertson has refused to speak publicly, saying only that he, too, is taking the time to reflect.
