Finance Minister Mathias Cormann is leaving Josh Frydenberg to explain why he has taken the treasurer's place for the G20 summit.
Senator Cormann will attend his fourth summit, travelling on Thursday with Prime Minister Scott Morrison to Argentina.
The pair will arrive in Buenos Aires in preparation for meetings starting on Friday.
Mr Frydenberg will instead stay in Canberra where the government is dealing with the fallout from Liberal MP Julia Banks quitting the party to become an independent.
Senator Cormann said it was a party decision for him to go instead of Mr Frydenberg.
"We made a judgement as a team that I should do what I have done on three previous occasions under Malcolm Turnbull and attend the summit," Senator Cormann told ABC Radio on Wednesday.
"(Mr Frydenberg) is the deputy leader (of the Liberal Party) and he's got some work to do domestically. I'll let him explain all of that."
Meanwhile, Mr Morrison won't get his first formal one-on-one meeting with US President Donald Trump at the summit.
A spokesman for Mr Morrison told AAP the prime minister would still have an opportunity to touch base with Mr Trump.
"But given we have no pressing bilateral issues at the moment, and the PM had an extensive opportunity with VP Pence at APEC, there is no pressing need for a formal bilateral at this stage," the spokesman said.
"The relationship is being well-managed."
