Bill Shorten has declared he can win an election fought on climate change whenever Tony Abbott calls one, but the embattled Opposition Leader must first clear a hurdle in his own party: a leadership-endangering move to rule out boat turn-backs under a future Labor government.

While the numbers at the ALP's triennial national conference in Melbourne looked set to swing further behind Mr Shorten on Friday, the Left faction has decided to hold off making a final decision about whether to push the boats issue to a vote.

A decision will be made on Saturday morning, just hours before that section of the platform is due to be debated.

With tempers running high, Labor's immigration spokesman, Richard Marles, received boos from delegates for his stance in favour of turn-backs but the Victorian hit back, warning Labor would lose if it persisted with a policy position based on emotion rather than results.

"Were we to go down a path of re-opening that journey [from Indonesia to Australia] again, you can forget about a future Labor government being remembered for anything else," Mr Marles said.

"We will be condemned by history. We would be consumed by that journey again, as we were when we were last in office."

Inside sources initially said the numbers among the 397 delegates were close, with many in the Left angry at what they saw as an ambush by Mr Shorten before the conference when he declared turn-backs should be open to a future Labor government.

However, as the time for the debate neared, key Left votes appeared to break away in a bid to sandbag Mr Shorten's position and ensure he is not defeated on the floor of the conference.

Senior Left faction frontbenchers Tanya Plibersek, Penny Wong, Brendan O'Connor, and Anthony Albanese were all expected to stand with their leader if their faction attempted to insert a prohibition on turn-backs in the party platform, which is currently silent on the question.

And a key union, the CFMEU, is also understood to have switched, with its delegates likely to line up with Mr Shorten in any vote.

The question of turn-backs is an incendiary one within Labor amid concerns about the humane treatment of asylum seekers and deep reservations about the Abbott government's hardline and ultra-secretive approach.

Day one of the conference also saw Labor ratchet up its objections to the yet-to-be-ratified China-Australia free trade agreement.

Trade spokeswoman Penny Wong said the text of the proposed FTA lacked promised guarantees over labour market testing in Australia before foreign workers can be brought in; contained inadequate controls over accreditation for trades such as foreign electricians, and included so-called ISDS provisions (investor-state-dispute-settlement) which will allow Chinese corporations to sue the Commonwealth if Australian policies in health and environmental protection, for example, limit their claimed right to commerce.

The opposition has indicated it will not attempt to stop the FTA coming into effect but Senator Wong flagged a Labor government would seek to renegotiate the above provisions and would use a scheduled two-year review of the FTA for that purpose.

That sparked fury in the Abbott government, with the Parliamentary Secretary for Foreign Affairs and Trade, Steve Ciobo, branding it a "retograde step by Labor".

"It would take trade policy back 30 years," he said.

Earlier, Mr Shorten used his opening speech to rally the troops, declaring Labor can win if it remains united and daring the Prime Minister to an election over climate change.

That followed Mr Shorten's decision to commit Labor to achieving 50 per cent renewable energy production within 15 years, first reported by Fairfax Media.

"We will win this fight," Mr Shorten told delegates.

"Let me say this to our opponents, in words of one syllable: an ETS [emissions trading scheme] is not a tax, and if Mr Abbott wants to make the next election a contest about who has the best policy solution for climate change, I've got a three-word-slogan for him: Bring it on!"

with James Massola

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