The ethnic composition of Australia's population will change if the fertility rate continues to run below replacement levels of 2.1 children per woman, Costello said while launching a federal government census.
Increasing the birth rate would cost taxpayers more in health and education spending and would create a short-term shortage of women in the work force, he said.
Australia's birth rate has increased in the past year to 1.8 children per woman from 1.73, following a steady decline from 3.55 in the early 1960s, according to the AP.
Australia was one of the few developed countries to record an increase in its fertility rate last year, the latest issue of Science magazine reported. Other nations, particularly Japan and Russia, are facing huge population declines in coming decades, it said.
Last month, Costello took credit for Australia's biggest annual baby boom in 13 years through his government's family-friendly policies.
Two years ago, he announced a 3,000 Australian dollar (US$2,300; Ђ 1,880) family bonus for each new baby born after July 1, 2004, and challenged Australians to "go home and do your patriotic duty tonight."
Australia's population is currently 20.6 million.