WA voters will emphatically endorse same-sex marriage in the Federal Government’s postal vote, a poll has found.

The ReachTel poll, conducted by The West Australian this week, found 63 per cent of voters agreed the law should be changed to allow same-sex marriage, with 37 per cent opposed.

More than 90 per cent of the 1723 people polled said they had returned their ballot paper or were very likely to when they received it in the post.

Labor voters (77 per cent) and Greens (93.8 per cent) overwhelmingly sided with the Yes case, but Liberal voters were split, with 50.3 per cent favouring same-sex marriage.

The gender split is stark, with more women (71.4 per cent) backing the change than men (54.6 per cent).

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Three-quarters of young people were in favour of the change, but support waned among older age groups, with over-65s favouring traditional marriage, 59.3 per cent to 40.7 per cent.

A Galaxy poll in July found 59 per cent supported same-sex marriage and 32 per cent were against it.

WA Yes campaign director Emma Gibbens said the level of support had been felt by volunteers.

“West Australians recognise that it is about fairness and equality, which are very Australian values so it’s very obvious to the West Australian public,” she said.

“We’re sensing a lot of support from our phone surveys. People are returning their ballot and are telling their friends and family to do the same.

“People are very engaged and they’re definitely responding.”

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In a blow to the No campaign, which launched in WA at a grassroots rally on Thursday night, more than two-thirds of those who did not support changing the Marriage Act said they would be unlikely to vote or would not vote.

Coalition for Marriage WA director Peter Abetz said the numbers were concerning.

“A big focus of the No campaign is to encourage people to vote,” he said. “I think it’s winnable but we will only be able to secure a no vote if everyone actively votes.”

Both sides are expected to increase their efforts, with a Yes campaign event planned for tomorrow.

The poll comes after a heated week in the same-sex marriage debate, with former prime minister Tony Abbott allegedly headbutted by a man opposed to his views.

AFL House in Melbourne was evacuated when a bomb threat was made after the organisation threw its support behind the Yes case. The threat was a hoax.