NSW Premier Gladys Berejiklian has defended her decision to allow an advertisement for a racing event be projected on the Sydney Opera House on Tuesday night, saying the image "will be in good taste".

Speaking on Monday morning on the decision that has sparked mass debate across the country, Ms Berejiklian said "all sides of politics" supported the decision.

She said the people should "wait and see" what images were used before they rushed to judgement.

"It is done in good taste," the Premier said.

"It's incredibly toned down from previous versions.

"It's important for us to promote our major events (and) I believe that tomorrow evening strikes the right balance."

The Premier's comments came as an online petition opposing the decision nears 200,000 signatures.

The petition, led by Sydney resident Mike Woodcock, calls on the NSW Government and Racing NSW to stop turning the Opera House into a "promotional billboard".

A mock-up of the projected barrier draw that will appear on the Sydney Opera House tomorrow ( Supplied: Racing NSW )

The promotion has sparked a wave of debate online and across the country, with some commentators questioning the appropriateness of racing, and its connection to gambling, being promoted on one of the country's best known landmarks.

Greens MP David Shoebridge urged the NSW Heritage Council to issue a stop-work order to prevent the advertisement which he calls "offensive".

"You have a duty to act to protect one of the state's most iconic heritage sites from being damaged for such a narrow commercial gain," Mr Shoebridge wrote in a letter to the council's chairperson.

He made note of the 2017 Sydney Opera House Conservation Management Plan which states:

"The exterior, particularly the shells, should not be regarded as a giant billboard or commercial/advertising opportunity."

Prime Minister Scott Morrison defended the decision, telling 2GB radio on Monday morning it was just "some lights flashing up there for a brief moment of time".

"It's not as if they're painting it on there," he said.

"That (image) goes all around the world and they do it for other things, the Wallabies, indeed and others.

"So look, I just don't understand why we tie ourselves up in knots about these things."

On Sunday he said: "Why not put [the event] on the biggest billboard Sydney has?"

A mock-up of one of the projection designs that was later rejected ( ABC News )

Opposition Leader Bill Shorten has attacked this characterisation of the world heritage building.

"I think I do actually speak for a lot of Australians [when I say] the Opera House is not a billboard," he said.

"It's a thing of great beauty, it is part of our national treasure and deserves the respect that comes with it."

Mr Shorten said he would like to remind Mr Morrison he is not a marketing executive.

The ABC understands Racing NSW is planning extra security at Tuesday night's event, with threats of protests reaching its offices.

Ms Berejiklian said the final approved design had been toned down from previous versions which had been published in the media.