The words "Vote No" appeared in Sydney's skies on Sunday, a day after the launch of Coalition for Marriage where high-profile Turnbull government MPs including Concetta Fierravanti-Wells said they fear it will become illegal to oppose same-sex marriage in word or even thought, if gay marriage is legalised.

"The 'yes' side want to make it illegal to just express a different view about marriage, that is their agenda," Matthew Canavan, a member of Prime Minister Malcolm Turnbull's cabinet until he resigned over his dual citizenship, told the 1500-strong Sydney audience.

Speaking to Fairfax Media he said he feared "a strong push to effectively eradicate the view that marriage should be between a man and a woman, to make it illegal".

The "Vote No" call to action appeared was visible from much of Sydney's inner suburbs.

A GoFundMe page claiming to support "Vote No" has so far raised more than $2300 in four days for the cause.

The reaction on social media to the skywriting was colourful.

A strong south-easterly breeze across the harbour erased the "Vote No" sign from the sky after only a few minutes.

The Yes campaign then officially launched in Melbourne with a number of actors and celebrities backing the cause.

Actor and activist Magda Szubanski fronted the event on Sunday which follows a number of other public functions that have been staged around the country to mobilise the Yes vote, including well attended rallies in Tamworth and Adelaide.

Speaking at a Sydney event, Christine Forster, a Liberal councillor, accused the No campaign of trying to confuse voters.

"The No campaign is trying to muddy the waters with issues that are not related," she said.

"This is a simple question of whether or not you believe the law in Australia should be changed to allow same-sex couples to marry. That is as far as it goes."

She later took to Twitter to condemn her brother, former Prime Minister Tony Abbott who is advocating a No vote, for claiming protections for religious freedom should have been put in place before the survey was launched.

"A plebiscite was your idea Tony Abbott. As a legislator if you thought protections were needed why haven't you put them in place?" she wrote.

The "vote yes for same-sex marriage" message has also made its way across the Nullarbor with the walkways outside WA's Parliament House being transformed into rainbows.

Last week Frances Abbott, the daughter of former Prime Minister Tony Abbott, pledged her support to the yes campaign.

Marriage equality is something she supported even while she was on the hustings with her father during his last election campaign.