Australia and New Zealand will have the fastest growing populations among developed nations for the first quarter of the next century, according to a United Nations Population Fund report. It said New Zealand's present population, 3.7million, would rise to 4.9million by 2025, a 36 per cent increase. Australia would be the next fastest-growing developed nation, with a forecast 29 per cent rise from 18.4 million to 23.9 million.