The NSW government is facing intense pressure over its decision to force Sydney Opera House management to promote a $13 million horse race with some critics saying the move is illegal.

A petition opposing the plan to project Tuesday night's barrier draw for the Everest race onto the venue's sails has garnered more than 206,000 signatures and protesters plan to disrupt the event.

National Trust NSW conservation director Graham Quint says projecting commercial material onto the Opera House contravenes state laws.

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"They've gone with a decision that we think probably is not legal," Mr Quint told AAP on Monday.

"It may be in breach of the Heritage Act."

Mr Quint says the legislation makes clear that projecting colours and images onto the sails should be "confined to exceptional, non-commercial occasions of brief duration".

The heritage expert added the Racing NSW promotion had been referred to the World Heritage body UNESCO.

Read more Berejiklian unmoved by backlash against Opera House 'advertising'

The venue's own conservation management plan explicitly states "the Sydney Opera House exterior, particularly the shells ... should not be regarded as a giant billboard or commercial/advertising opportunity."

"This is a black and white issue," state Greens MP David Shoebridge told AAP.

"It's a classic Sydney political story that has ignorance teaming up with special interests to once again sell out the public."

Premier Gladys Berejiklian on Friday intervened to allow the advertising after Sydney shock jock Alan Jones publicly berated Opera House chief executive Louise Herron who'd ruled against Everest words or branding being projected onto the venue because "it's not a billboard".

The premier on Monday stood by her decision saying she was "incredibly comfortable".

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"We would never do anything that wouldn't support the iconic stature of the Opera House but it is also extremely important for us to promote those events that bring jobs and economic activity to NSW," she told reporters.

Prime Minister Scott Morrison also refused to back down after calling the Opera House "the biggest billboard Sydney has" on the weekend.

"It's not like they're painting it (the promotion) on there," the Liberal leader said on Monday.

Federal Opposition Leader Bill Shorten was quick to attack the PM stating: "The Opera House is not a billboard, it's a thing of great beauty."

"It's part of our national treasure and deserves the respect that comes with that," he said in Melbourne.

Former venue chief executive Michael Lynch argues promoting a horse race is a "crass, inappropriate and offensive use of the Opera House sails".

With the debate heating up, Racing NSW chief executive Peter V'landys on Monday revealed he and his staff had received death threats.

"There have been death threats and acts of violence - there's no need for that," he told 2GB.

"If you want to protest peacefully by all means (do so). But do it in the right way."

Protesters will gather at the Opera House on Tuesday night to try and disrupt the Everest projection using torches and mobile phone lights.

"The more people we have the more light we will be able to shine on the Opera House," organiser Rachel Evans told AAP.