But the policy will kill off any hope of a Labor resurrection in the seat of Melbourne, held by the Greens' Adam Bandt since 2010.

Monash University political expert Paul Strangio said the policy was a clear sign of Labor's focus on seats in NSW and Queensland at the next election.

"Every time Labor has endeavoured to narrow the policy gap with the Coalition on asylum seekers there is little evidence it has done them any favours politically," Dr Strangio said.

But how much the policy swings more progressive votes the Greens' way is unclear, with both ALP and Greens officials saying many voters who traditionally voted Labor on refugee policy had already switched sides.

Dr Strangio said the big challenge for Labor was whether they could sell the nuanced part of the policy such as regional processing and a boost to the humanitarian intake.

"Do they have the capacity to deliver this? I doubt there is the capacity because the public debate [on asylum seekers] is so hopelessly debauched," he said.

For many in Labor against Mr Shorten's policy the issue is about maintaining socially progressive values and telling voters about this.

Dr Strangio and Labor campaigners in Victoria say the policy is unlikely to hurt Labor in four seats it is working to win from the Coalition; Corangamite, Deakin, La Trobe and Dunkley.

The Greens have been making in roads in inner northern Labor-held seats of Wills and Batman, but the consensus is that Labor should retain them at the next poll despite Mr Shorten's new policy on boat arrivals. The Greens fancy their chances in Batman ahead of Wills.

Senior ALP officials concede the policy will snuff out any hopes of a comeback in Melbourne, saying the primary vote was already at rock bottom.

In the four seats Labor can win it will have little impact, with party research showing that the asylum seeker issue had little influence in suburban seats in Melbourne, with voters more concerned with jobs, education and health.

Senior Greens sources say whenever Labor moves to the right it helps their vote, although they were quick to say that "this is not the first time Labor has been nasty to refugees".

They conceded that the number of people shifting from the ALP to the Greens on this issue may have slowed.

"What the policy does do was reinforce the Greens brand as a party that made ethical policy decision, not one based on the polls and pragmatic politics," one said.

Labor's candidate for Melbourne, Sophie Ismail, in a statement on her Facebook page, declared that she did not support unilateral boat turnbacks.

"I do not support the degrading treatment of asylum seekers in detention. I do not support policies which blatantly breach our obligations under international law. I support a greatly increased humanitarian intake and children out of detention immediately," Ms Ismail said.

"I will not vote against my conscience or the conscience of this electorate on these issues."