Labor will pull out all the stops and campaign for a "yes" vote in the looming postal plebiscite on same sex marriage, Bill Shorten will confirm, putting to bed suggestions the federal ALP could boycott the vote.

Mr Shorten is due to announce Labor's plan to campaign for a "yes" vote in a speech after question time on Thursday.

The speech will mark the formal confirmation of Labor's position, which has been hinted at by members of the frontbench, and it has not been taken lightly.

The ALP held a series of meetings with same-sex marriage advocates on Wednesday to discuss tactics and strategy about the campaign. It has also been discussed at a series of working groups that included the ALP leadership group and other MPs on the front line of the campaign for marriage equality, such as legal affairs spokesman Mark Dreyfus and shadow assistant minister for equality Terri Butler.

The Opposition Leader will in his speech attack the plebiscite process, arguing it was designed by people who don't want same-sex marriage to be legalised and will call it a waste of money.

Nevertheless, Mr Shorten will also say he is a strong supporter of same-sex marriage and will fight for it in any forum.

A source close to the Opposition Leader said that if the plebiscite process couldn't be killed off Mr Shorten will want to campaign hard to make sure the "yes" vote wins.

"It's a shame the Prime Minister is too chicken to do the same. Turnbull will be responsible for every piece of offensive and hateful material that this campaign generates – he has given it licence. We will make sure everyone knows this". 

Labor's decision to campaign for a "yes" vote comes despite the federal opposition opposing the postal plebiscite, which will be conducted by the Australian Bureau of Statistics, not the Electoral Commission, and cost $122 million. 

Labor Senate leader Penny Wong delivered a blistering takedown of the postal plebiscite on Wednesday.

Marriage equality campaigners Just.Equal and PFLAG have announced a High Court challenge to the plebiscite, and the Equality Campaign is still considering a similar challenge, pending legal advice.

But advocates for the law change have told Fairfax Media that, while the attempt to block the plebiscite will go ahead, "the minute Labor, the unions and business get into this" it will be almost impossible for members of the gay community to stay out of it.

"People are starting to move on already," a "yes" advocate, who asked not to be named, said.

" This has been foisted on us, we don't like it, but we have to get people on the register so they can vote 'yes'."

The Greens, similarly, have not yet said if they will boycott the plebiscite and like Labor, have held discussions with stakeholders.

However in a statement released Thursday, Greens leader Richard Di Natale said that supporters of same-sex marriage should enrol in the event the vote goes ahead.

"Enrolling to vote is a win-win. We are hoping that the shonky postal plebiscite is knocked off by the upcoming court challenges, but you'll want to be on the roll in case it isn't. And even if the plebiscite is knocked off, you'll want to be able to vote to kick this government out at the next election for playing politics with our human rights," he said.

The key dates for the plebiscite are September 12, when the Bureau of Statistics begins posting out ballots; November 7, when responses have to be received and November 15, when the result will be announced. 

If a "yes" vote is successful, there will be two weeks left before the end of the parliamentary year for a vote to take place in the Parliament.

Mr Turnbull said on Thursday that his government would not "facilitate the introduction of a private members' bill on this matter unless the Australian people have given their support through a 'yes' vote through this national vote
that we are now undertaking".

Former Liberal prime ministers Tony Abbott and John Howard have already indicated they will play a prominent part in advocating for a 'no' vote.

Mr Abbott has said Australians should reject same-sex marriage if they want to protect religious freedom and reject political correctness.

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