Federal Opposition Leader Tony Abbott has defended the monarchy against republican commentators, saying the monarchy is "evolving and renewing" through the recent royal engagement.

Speaking at the national conference of Australians for Constitutional Monarchy in Sydney on Saturday, Mr Abbott acknowledged that this "natural" renewal of the monarchy through the engagement of Prince William and Kate Middleton was not done as other institutions might, "through legislation, votes or upheaval".

Instead, he noted, it was done through "something as natural as the marriage of an appealing man and an attractive woman".

In his defence of the monarchy against republican commentators, he said there was a gap between the mainstream and the commentators who "can't quite get an institution with its roots in an earlier time and a different way of thinking".

He also admitted that the monarchy is discriminatory and that republicans would focus on this.