Malcolm Turnbull is set to give a speech at the 25th anniversary dinner of the Australian Republican Movement.
The prime minister, a founding member and former chair of the movement, has agreed to attend the event at Sydney University on Saturday night.
"It is an occasion to honour those who've got us to this point, and the PM is, of course, at the forefront of our founding fathers and mothers," national chair Peter FitzSimons said in a statement on Monday.
The Australian Monarchist League warned Mr Turnbull would lose more voters to minor parties like One Nation by actively supporting the republican movement.
"Malcolm Turnbull is forgetting the reality of politics," chair Philip Benwell said.
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"We know that more coalition voters support the constitutional monarchy than not - they are prepared to tolerate Malcolm Turnbull because he has made the proviso 'not yet'."
