Environment Minister Josh Frydenberg insists a half billion-dollar grant awarded to a small Great Barrier Reef charity is "not unusual" as the opposition demands answers.
Labor senator Kristina Keneally is leading a charge against the Turnbull government over the "curious" $444 million taxpayer-funded injection to the Great Barrier Reef Foundation.
Mr Frydenberg said the grant was contained in a budget submission he brought to cabinet but refused to clarify whether it was his idea.
"This was not an idea developed overnight," he told the ABC.
"It goes back to early last year when my department chaired an inter-departmental task force looking at how we meet the challenges from the reef."
Mr Frydenberg would not be drawn on what steps were taken to ensure the foundation was the best candidate for the public money.
"There was extensive due diligence," he said.
The grant was handed over to the foundation in one lump sum.
"It is not unusual for governments to do that," Mr Frydenberg said.
"The money is to be spent over six years. By giving it all at once, they have maximum leverage to enter into contracts and start providing the money as needed, as they meet their objective."
Senator Keneally believes the grant was handed over in a mad rush earlier this year.
"This is a strange way to administer public money," she told Sky News on Sunday.
"Labor will continue to pursue this through the Senate inquiry and every other avenue available to the opposition."
Labor has called for the grant to be handed back to taxpayers and managed with probity.
Australian Associated Press