In exclusive comments to Fairfax Media, Mr Shorten has hardened his criticism of the plebiscite plan in a clear indication Labor is preparing to announce it will not support the Coalition's enabling legislation.

"I'm worried Malcolm Turnbull will just stuff it up," Mr Shorten said. "He stuffed up the republic referendum, he stuffed up the NBN and he stuffed up Senate reforms when he promised to fix it."

Mr Shorten's leadership group discussed the issue behind closed doors this week. While it's understood they did not reach a final decision – and the issue may still be put to the full Labor Party room – they are increasingly inclined to vote it down.

While this position carries political risks, given Labor strongly supports same-sex marriage, there are concerns in Labor's ranks a plebiscite will fail.

They fear Mr Turnbull will put no effort into the "yes" campaign – which is split on the merits of the public vote – allowing the well-organised and well-resourced "no" campaign to steal a march and ultimately carry the day. If that happens, it will make it more difficult for a future Labor government to legislate for it.

Labor hopes its position will increase the pressure on Mr Turnbull to defy his party's conservative wing and allow a free vote in the Parliament.

However, Coalition sources insist that is highly unlikely given Mr Turnbull's tenuous hold on the House of Representatives.

Mr Shorten's latest comments come after the Greens committed to voting against the government's enabling legislation. Greens leader Richard Di Natale said people's rights should not be subject to a popular vote that could unleash hatred.

Without Labor or Greens support, Mr Turnbull would have to rely on the crossbench to get it through. One Nation has signalled it will support the plebiscite but Nick Xenophon and others want a free vote, leaving the government short.

Mr Turnbull doesn't even support a plebiscite, Mr Shorten said.

"He's only doing it because he is too weak to stand up to the radicals in his own party," he said. "Why should everyone else have to pay for his weakness?

"The plebiscite is unnecessary, expensive and divisive. There's a better, faster way to make much marriage equality a reality. The Parliament should do its job and deal with a marriage equality bill, with all parties afforded a free vote."

Labor is also concerned the plebiscite will cost much more than the oft-quoted figure of $160 million. Sources say the true cost may be closer to $250 million.

There are concerns, too, the plebiscite will stretch the Australian Electoral Commission to breaking point.

There are further worries the result can be heavily skewed if the vote is not compulsory as highly motivated opponents are ready to capitalise on the possibly mistaken perception that the plebiscite will easily succeed.