The actor Margot Robbie has made a high-profile stand for marriage equality in her home country of Australia while hosting Saturday Night Live.

Robbie appeared on the season 42 premiere – SNL’s highest-rating opener in eight years – wearing a T-shirt that read “Say ‘I Do’ Down Under”, with a map of Australia in rainbow colours.

The T-shirt is part of a new campaign by the British actor Joshua Sasse (who is engaged to Kylie Minogue) to legalise marriage equality in Australia.

Sasse launched the campaign at the weekend to coincide with Robbie’s appearance on SNL, posting an image of himself wearing the shirt alongside Minogue and Dolly Parton.

He also shared images of Jake Shears of pop group Scissor Sisters and the US actor and talk show host Kelly Ripa both wearing the design.

“I was shocked to find out earlier this year that same-sex marriage in Australia is still illegal,” he wrote on the campaign’s website. “There are many hurdles to jump in order to make constitutional change, but we are now the closest we’ve ever been to that change.

“Let’s raise our voice and public support for change. Get a shirt. Wear your heart on your sleeve.”

The shirts are in support of Australians 4 Equality, a coalition of organisations that believe marriage equality should be reached through a free vote of parliament rather than a plebiscite. Newspoll last week found that support for a plebiscite has dropped from 70% to 39%, with 48% of Australians now supporting a free vote.

The shirts are not yet available for purchase, but those interested can sign up to a mailing list.

In September Minogue told News Corp Australia that Sasse “loves Australia”, but was at a loss to explain why marriage equality was not legal.

“He’s so adamant to fight for gay rights in Australia and it’s coming from the most genuine place. He just can’t fathom that same-sex marriage hasn’t been legalised and of course I back him up on that, we are waiting for it to happen.”

The Say ‘I Do’ Down Under shirt was one of many outfits worn by Robbie over the course of the SNL premiere, which included a dress patterned with marijuana leaves.

The episode also featured Alec Baldwin’s debut as Donald Trump, and Robbie’s impression of his daughter Ivanka.

According to Nielsen figures, it was SNL’s most-watched premiere in eight years, since Tina Fey made a guest appearance as Sarah Palin in September 2008.