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. 

The reaction on social media was colourful.

​It is not yet known who funded the advertisement but thanks to a strong south-easterly breeze, "Vote No" was erased after only a few minutes.

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

Actor and activist Magda Szubanski fronted the event 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.

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, a vocal supporter of the no vote, 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.

The story, 'Vote no' sign written in sky above Sydney ahead of Yes launch for same-sex marriage survey, first appeared on the Sydney Morning Herald.