The High Court has ruled 7-0 to dismiss challenges to the Federal Government's $122 million same-sex marriage postal survey, clearing the way for it to go ahead.

Key dates in SSM postal survey: August 24 — the final day to register with the AEC if you want to take part in the survey

September 12 — survey forms start being sent out

September 25 — all forms are expected to have been sent

October 27 — forms are strongly encouraged to be returned by this date

November 7 — the final deadline to return surveys

November 15 — results are released

The Federal Government welcomed the unanimous High Court ruling, announcing the survey would continue as planned and the final result would be declared by the chief statistician at 11:30am on November 15.

Now that it has been given the green light, the Australian Bureau of Statistics will start sending out ballot papers from Tuesday. Everyone should have received their survey form by September 25.

The paper will ask: "Should the law be changed to allow same-sex couples to marry?"

Returning the survey and participating in the ballot is optional, but must be returned by 6:00pm on November 7 at the latest in order to be counted.

Australians to have their say on 'important social question'

Attorney-General George Brandis announced the High Court's decision to the Senate.

"The effect of the decision of the court is that there is now no legal impediment to that postal survey proceeding and all Australians having their say on this important social question," Senator Brandis said.

Sorry, this video has expired George Brandis announces High Court decision on postal survey

Prime Minister Malcolm Turnbull urged all eligible Australians to vote in the survey.

"This is a great example where every Australian can have a say and we can, as a Commonwealth of Australia, embrace this important social change, consider it and make a decision," Mr Turnbull told Parliament after the decision was handed down.

Opposition Leader Bill Shorten again invited Mr Turnbull to sign a joint letter with him recommending a vote for "marriage equality".

Mr Turnbull indicated he was sceptical co-signing a letter, "would increase the case for the Yes vote".

"The Leader of the Opposition can make his case and I will make mine," the Prime Minister said.

The Greens said they were disappointed with the result in the High Court but declared they would now campaign hard for a Yes vote.

All seven High Court judges ruled in favour of the Federal Government in both of the challenges under consideration.

The judges also ordered that those responsible for the unsuccessful challenges had to pay the cost of the cases.

The first group was led by Tasmanian independent MP Andrew Wilkie, PFLAG (Parents, Family and Friends of Lesbians and Gays), and Melbourne mother Felicity Marlowe.

The second group involved same-sex marriage advocates Australian Marriage Equality (AME) and Greens senator Janet Rice.

The groups argued Finance Minister Mathias Cormann did not have the authority to appropriate funds for the postal vote because it was not an "urgent" or "unforeseen" matter.

They also argued the Australian Bureau of Statistics had unconstitutionally been directed to seek an opinion rather than "statistical information".

Mr Wilkie expressed his disappointment after the announcement, insisting the survey was unlawful.

"Regardless of the legality, this is and always was bad government policy," he said in a statement.

"We've already seen how divisive and hurtful this postal vote can be, and the best solution remains for parliamentarians to just do their jobs and vote on the issue.

"I respect the court's decision, however, and so my focus now turns to making the most of the postal vote. I'll be voting yes and will be encouraging everyone else to do the same."

Senator Rice also urged people to vote yes in the survey.

"It is very clear what the path forward from here is and that's to really continue to bring the whole movement together to get so many people out, getting people to choose love and vote yes," she said.

No campaigners welcome High Court decision

Lyle Shelton, a No campaigner from the Australian Christian Lobby and the Coalition for Marriage, welcomed the High Court's decision to allow the survey to go ahead.

"I'm confident in the judgement of the Australian people," he said.

"I think when they fill out those ballot forms around the kitchen table, around the nation, without being pressured or made to feel like they are somehow bigots or lesser citizens because they might hold a different view, I think that will be the real poll.

"That is the test of what the Australian people think."

Sorry, this video has expired Coalition for Marriage's Lyle Shelton concerned Yes vote would harm free speech

Former prime minister Tony Abbott, who has said he would vote no in the postal survey, shared a video on Facebook saying he was glad Australians would "finally have their say on marriage".

"It's a real pity that some supporters of same-sex marriage wanted to deny the Australian people their say on this vital topic. Nevertheless, the High Court has spoken, and the postal plebiscite will go ahead and I very much hope you will vote to protect the family."

What happens after the postal survey?

If the result of the postal survey is no, the Prime Minister has said the Government would not support a private members' bill on the issue going ahead in Parliament.

But if those who participate in the survey vote yes, then Mr Turnbull has said a private members' bill could be debated and voted on by the end of the year.

The Government has also said it would move to legislate to ensure advertising for the Yes and No cases met the usual electoral rules, including protections against malicious publications and bribery.

The High Court's ruling means the advertising for both sides will intensify.

Shadow attorney-general Mark Dreyfus said he expected a bill to provide those protections to be introduced next week.

Mr Drefyus has been negotiating with the Government on that bill.

He said he wanted it to go further than the ordinary electoral protections.

"The Government has said it will base the safeguards on existing provisions in the Electoral Act, we don't think that those protections go far enough," Mr Dreyfus said.

"We don't think that they are particularly appropriate for an absolutely unprecedented, unusual, unnecessary postal survey but we'll see what the Government is prepared to go with. Again, I say Labor wants these safeguards to be as strong as they can possibly be."

Greens leader Richard di Natale and senator Janet Rice spoke to the media after the High Court decision. ( ABC News: Matt Roberts )

Greens leader Richard Di Natale said his party would seek to have stronger protections than are normally in the Electoral Act.

"We have long campaigned for truth in advertising during election campaigns and we will be making sure we do what we can to have strong protections during this postal survey," Senator Di Natale said.

There will also be pressure on federal parliamentarians to explain what they will do if Australians vote yes.

So far, some MPs have pledged to "respect" the result of the survey but have not said whether that meant they would vote according to the overall national result or instead back the result from their electorate or state.