Support for an Australian republic has collapsed to a 20-year low, with just 39.4 per cent of Australians saying they support a republic.

Support was lowest among older Australians and Generation Y voters, with people aged 35 to 65 most supportive of abandoning the monarchy.

Governor-general-designate General Peter Cosgrove. Credit:Mal Fairclough

An exclusive ReachTEL poll of more than 2100 Australians, conducted on Thursday night for Fairfax, shows 41.6 per cent oppose the country becoming a republic; 19 per cent had no opinion on the issue.

Australians for a Constitutional Monarchy national convener David Flint said the findings were a ''time bomb'' for the republican movement, with support among 18 to 35-year-olds at 35.6 per cent. Only those over 65 had a lower rate of support (30.7 per cent) for a republic.