Federal Environment Minister Josh Frydenberg has refused to answer why there was no tender process for a record $444 million grant to the Great Barrier Reef Foundation.
"It was a good idea ... there's nothing to hide here," Mr Frydenberg told radio 2GB on Thursday.
"Why we chose this organisation was because it was pre-eminent in raising funds from the private sector."
The foundation says it has raised $90 million but Labor environment spokesman Tony Burke says its annual reports don't add up to the figure.
Labor has called for the funding to be returned and accused the charity of lying about how much money it has raised in its 18-year history.
Mr Burke noted the foundation's website changed late on Wednesday night to say it had raised 64 per cent of the $90 million from corporate and private philanthropy.
"The claims that the foundation has been making, that they were raising all of this money from private sources, are simply not true," Mr Burke told reporters in Brisbane on Thursday.
"This entire gift - of the privatisation of the responsibility of the Great Barrier Reef - has been based on a lie."
The funding was announced in April, with a Senate inquiry since revealing the money was offered to the charity in a private meeting with Prime Minister Malcolm Turnbull and Mr Frydenberg.
The foundation's managing director Anna Marsden told the inquiry last week the money was a surprise and the group "did not suggest or make any application" for it.
The prime minister continues to insist awarding the $444 million was the result of a "very thorough" process and says Australians who care about the health of the reef should welcome the news.
"It went through a full cabinet process, it was in the budget, it was in the appropriation bills that the parliament has passed so it has been legislated," he told reporters in Perth on Thursday.
"It will ensure we get this substantial amount of money into maintaining the health of the reef."
The auditor-general has listed the grant as a "potential" inquiry in its 2018/18 work program but has yet to commit to a full audit.