Paul Keating has slammed the proposed expansion of the Art Gallery of NSW, calling the project a "land grab" entertainment complex "masquerading as art".

In an opinion piece for The Sydney Morning Herald on Wednesday, the former prime minister accuses gallery director Michael Brand of constructing "a megaplex made feasible only by appropriating open space across an expressway and on land belonging to the Botanic Gardens".

The proposed $450 million Sydney Modern project would see a series of low-rise, glass-walled pavilions with harbour views built across the land bridge and harbourside of the Cahill expressway and into land belonging to the gardens.

"Brand has spent most of his term in office constructing a gigantic spoof against the civic core of Sydney's most public and important open space" Mr Keating writes.

The expansion would capitalise on expansive harbour views from a function centre and observation platform for commercial use, according to the project's website.

"In other words, the big-to-do is all about idolatry of special events and the provision of commercial venues for hire, rather than about curating or exhibiting art," Mr Keating writes.

In October Mr Brand told The Australian Financial Review: "The function room has to be in a nice position, so you have views over Woolloomooloo perhaps, and it needs to be separate from the galleries so that when it's being set up it doesn't disturb visitors.

"But it also needs to feel like it is part of an art gallery, so it can't be too remote," Mr Brand said.

But Mr Keating said the revenue-generating private functions pavilions was about "money, not art".

"Decoded, Brand is telling us he proposes to build a large entertainment and special events complex masquerading as an art gallery," Mr Keating writes.

The project is yet to secure funding, however in May the gallery announced Japanese firm SANAA had won the architectural competition to design Sydney Modern.

"We're getting good support from the state government so far. We are never going to have a better chance. No guarantees, but it looks very good," Mr Brand told Fairfax Media.

"Brand thinks he can push the system hard enough and with a bit of luck, suck the government into the funding, his mega-dream can become a reality," Mr Keating writes.

Mr Keating backed an alternative expansion of the gallery to the south over the existing Domain carpark, which Mr Brand rejected in 2014, saying "an expansion to the south would have threatened the heritage value of our southern facade as well as a number of heritage trees".

Keating has a long history as both outspoken critic and advocate of ambitious public works in the state's capital.

The headland park at Barangaroo is said to be a testament to his single-minded determination to recreate the original naturalistic headland destroyed 100 years ago by maritime development, despite unsuccessfully lobbying for James Packer's casino and hotel to be moved to Hickson Road.

His vision for Sydney would see Goat Island returned under native title to the original owners, and his failed attempt to bury the Cahill Expressway at Circular Quay was reinvigorated last week with AMP urging the NSW government disguise the notorious eyesore.

In April last year he told ABC Radio 702 listeners that you would have to be "brain-dead" to build anything at Mrs Macquaries Point, taking aim at Sydney Parklands and Botanic Gardens chief executive Kim Ellis over the $130 million master plan for the space.

In the same interview called Brand "just another property tart" over the Sydney Modern project.