My second challenge: Big Australians need to get honest about the intensified zonings required as both Sydney and Melbourne climb to 7 million by mid-century.

Business lobbies and conservative think-tanks want the capitalist free-for-all that goes with rip-roaring numbers. But they don't want any hike in public spending, tax and borrowing. But face the fact: high immigration brings either congestion or the heavy spending required to fix it.

Not one of the Big Australians – the Business Council for example – has said that for every additional 25,000 migrants above a certain benchmark in any capital the commonwealth must fund 10 kilometres of inner city underground or light rail or bus transit-way. In turn, the commonwealth might require the states to reserve corridors. State and commonwealth governments should be able to guarantee Australia would not be laying out new suburbs that leave residents beyond easy walking distance from public transport.

Right now, they wave it away, saying "Oh it's all a matter of infrastructure", as if this had never been thought of. But even massive infrastructure spends by states is only keeping pace with 80,000 to 100,000 added to both Sydney and Melbourne each year.

An extra few million for each of our biggest cities will mean towers and towers and more towers. Credit:Jim Rice

Former New South Wales premier Bob Carr says Big Australia advocates are failing to answer the questions their cause raises. Credit:Alex Ellinghausen

Lack of candour on density was exemplified by former NSW premier Barry O'Farrell. He declared his support for ambitious immigration figures. But his biggest planning decision as premier was to kill off plans for 10,000 new units through rezoning along the north shore rail line in Ku-ring-gai. Placing towers along a transport corridor (only six per cent of the council area) was sound planning. With such dishonesty the joyful rhetoric about a bigger Australia is separated from discussion of the urban form needed to support it.

Chris Johnson, the former Government Architect now with the Urban Taskforce, has drafted maps that show thick forests of towers around transport nodes like Hurstville or Parramatta. He hopes the next step is an underground rail system linking them. This is the kind of honesty I want. But two concerns open up: first, we already have thick forests of residential towers. How much thicker might they be, at Chatswood or Strathfield for example, to accommodate a city of seven million? And, second, what would be the time lag before the world's most extensive metro, an underground running from Bondi to Parramatta, gets installed?

Harry Triguboff is a sincere advocate of a Big Australia, probably none more. In 2006 he suggested that Sydney's green areas should be opened up for development because there were too many forests and parks. This vision of his elegant towers above the waters of Port Hacking or Pittwater was cheekily intended to provoke a premier proud of his parks.

So in a similarly playful spirit I advance my own modest proposal: a re-zoning of Point Piper to lift its population from its current 6000 to a robust 30,000, pumping up its R2 zonings to allow stepped towers rising from, say, five stories on Wolseley Crescent reaching 30 in Wunulla Road. Its most famous resident at number 46 declared, before he became Prime Minister, "Density is not the problem it's the answer." The state's battering-ram planning laws would surely not be required to install these towers since Point Piper's residents contain many business leaders attached to the Big Australian vision and – I assume – eager to help accommodate the 50 to 100 million they long for so ardently.