Half of Australian voters support a 90-day limit on holding asylum seekers in offshore detention on Manus Island and Nauru, but Malcolm Turnbull isn't one of them.

Just 30 per cent of people were against the idea when surveyed for the Sky News ReachTel poll, while one-in-five voters were undecided.

Labor is preparing to debate offshore detention and the treatment of asylum seekers at its national conference in July.

The draft party platform, released last week for comment, calls on a Labor government to "strive to ensure (detention) is for no longer than 90 days" and seek third-country resettlement deals.

"Detention in an immigration detention centre is only to be used as a last resort and for the shortest practicable time," it says.

Labor frontbencher Penny Wong said the party should be advocating for detention for processing purposes to be limited.

"We don't have a specific limit in our policy currently, and these will be matters that the national conference will discuss," she told Sky News on Thursday

Senator Wong, a key figure in the party's Left faction, said offshore processing was never intended to serve as a vehicle for indefinite detention.

The prime minister said if Senator Wong was supporting such a limit she would be "rolling out the welcome mat to the people smugglers".

"It is extremely irresponsible," he told reporters in Nowra, on the NSW south coast.

"We must maintain the security of our borders."

Cabinet minister Mathias Cormann said Labor was "playing with fire", having taken a political hit in 2013 over asylum seeker policy.

"They're still wanting to put people smugglers back into business should they have the opportunity to get into government."

Labor immigration spokesman Shayne Neumann told AAP the party would never let people smugglers back in business.

"We do not have a specific limit proposed for offshore processing, but that doesn't mean Manus Island and Nauru should be places of indefinite detention, which is what they have become under Abbott and Turnbull," Mr Neumann said.

"Labor in government will prioritise establishing durable and suitable third country resettlement agreements."

Support and disapproval levels for the 90-day limit were the same across coalition and Labor voters.

Senator Wong said it was a Labor principle that offshore processing be temporary.

"It has been really appalling the way in which Peter Dutton and others in this government have used offshore professing as an indefinite detention."