Chris Bowen is due to spend more than $150 million on the United Nations' 31st Conference of the Parties meeting, known as COP31, in his role as president of negotiations.
While Australia is not hosting the annual summit, it is chairing the talks with Pacific nations as part of a diplomatic compromise with Turkey struck in 2025.
But opposition energy spokesman Dan Tehan said there were issues with the cost of chairing the talks, labelling it a vanity project, and that there were concerns with international travel expenses racked up during a cost-of-living crisis.
"Chris Bowen has serious questions to answer over this budget blowout," Mr Tehan said in a statement.
"Chris Bowen is spending taxpayer dollars to pump up his own tyres."
Mr Bowen was quick to tell reporters on Tuesday that Mr Tehan was "the biggest hypocrite in the federal parliament", criticising his counterpart's travel.
"I do note that I've taken one international trip this year. So has Mr Tehan. I went to Denmark, he went to Taiwan," he said.
"In his last year as minister, he went to France, Singapore, Vietnam, Korea, Japan, the United States, Indonesia, India, the Maldives - twice - at taxpayer expense."
In a Senate estimates hearing, Environment Department officials told Liberal senator Sarah Henderson the budget had allocated funding for 70 full-time equivalent staff ahead of the UN-run conference in November.
Australia is understood to be working closely with Pacific partners Tuvalu and Fiji, who will host pre-meeting in the lead-up to the main event, held in Turkey.
Earlier, Mr Bowen told ABC Radio that Australia was investing in COP31 to elevate its global influence.
"These things do cost money. It cost money when John Howard chaired APEC, it cost money when Tony Abbott chaired G20. They were good for the country and the Labor Party supported them, because we're a patriotic party," he said.
The minister said the cost of Australia hosting the G20 summit in 2014 was $400 million, while taxpayers were billed $350 million when Sydney held APEC in 2007.
