Concept images show the proposed northward expansion of the art gallery. Credit:Kazuyo Sejima and Ryue Nishizawa/SANAA What's more, Brand's brochure goes on to tell us that the new building "will create expansive …outdoor spaces". And he is certainly right about expansive. The footprint of the proposed building is utterly breathtaking in its scale. But what the brochure doesn't tell us is that current outdoor shaded space is to be replaced by outdoor unshaded space – across vast, flat, unprotected concrete terraces. The desire to extend the gallery northwards arises purely from institutional conceit and cultural snobbery about the harbour; rejecting the logical solution of building to the immediate south over the existing Domain carpark – a football field-sized parcel of land open to a higher cultural use. Notwithstanding this, any new galleries that might arise from Brand's proposed development will have been designed from the outside in rather than from the inside out. The models of the proposed extension which have been on display at the Art Gallery do not delineate any contained gallery spaces.

Art Gallery of NSW director Michael Brand said an enhanced capacity for revenue-generating private functions underpins the proposed Sydney Modern project. Credit:Dallas Kilponen Rather, they show big expanses of unrelieved plate glass intersecting large open slabs of concrete in terraced floorplates. And Brand doesn't hide his intentions. He recently told the Australian Financial Review magazine that "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 ... but it also needs to feel like it's part of the gallery, so it can't be too remote". Decoded, Brand is telling us he proposes to build a large entertainment and special events complex masquerading as an art gallery. The article went on to say "an enhanced capacity for revenue-generating private functions features prominently in the business case".

Well, of course, it does. This is what it's all about – money, not art. The Art Gallery of NSW is an arts institution; it is not a function centre or an observation platform. Nor is it a retail outlet. It is an arts museum and deserves to retain the integrity of a museum. So rather than focusing on new commercial revenue, Brand would do better to run the gallery optimally. This year attendances are 1 million fewer than the National Gallery of Victoria. Another million in attendances would do wonders for any real revenue problem Brand might believe he has. As to the plan itself, Brand weakly skirts around the logic of extending to the south by saying such an expansion "would threaten the heritage value of the gallery's southern facade as well as a number of trees". Yet he is proposing to obliterate the gallery's northern facade and wipe out a substantial chunk of the Botanic Gardens. And more than that, building a swollen lump of his megaplex on the bridge across the expressway. Imagine how bloated this is going to look and how disruptive it will be to all nature around it. And his claim as to the southern facade is not even plausible as the grassed rooftop of the Domain carpark is well to the east and well below the southern gallery facade. The carpark site is not even on Art Gallery Road – it is well separated from the gallery on Sir John Young Crescent.

Brand has been offering this weak apologia only to bypass the logical and less glamorous southern option – simply to build a case for his unfathomably vast northern extension. When the NSW government is out urging people to support an increase in the GST, Brand has the begging bowl out for $480 million for his mega-scheme. The government would be out of its mind to slug ordinary people 50 per cent extra on the GST to, among other things, fund the wayward ambitions of a gallery director. And all the carry on about his architects and their eminence represents nothing other than public pressure on the NSW government for funding. Unlike major international galleries – the Tate, the Victoria and Albert Museum, the Louvre, the Pompidou, all of which have opted to decentralise their central campus – Brand is determined to buck the accepted wisdom and insist upon a single-campus megaplex. Brand thinks if he can push the system hard enough and with a bit of luck, sucker the government into the funding, his mega-dream can become a reality. Sydneysiders need to be vigilant in defending their open space from this completely unreasonable assault.

Paul Keating is a former prime minister.