MELBOURNE'S population will grow to almost 7 million by 2049, federal Treasury has projected, doubling in the space of 50 years as an unending mining boom brings more and more migrants into Australia.

But revealing the figure yesterday, Treasury secretary Ken Henry voiced fears that Australia might be unable to handle another 13 million people by 2049 without serious environmental losses. ''With a population of 22 million, we haven't managed to find accommodation with the environment,'' Dr Henry told an audience in Brisbane.

''Our record has been poor, and in my view, we are not well placed to deal effectively with the environmental challenges posed by a population of 35 million.''

In a speech of outspoken frankness, optimism and gloom at the same time, Dr Henry argued that Australia was poised to ride a minerals boom that would last for decades, as India and China grew inexorably towards achieving Western standards of living.

But the Treasury chief, who is passionate about Australian wildlife and the bush, warned that the changes ahead ''will test the limits of sustainability: economic, social and environmental''.