Labor's scrutiny of the controversial $444 million grant to the Great Barrier Reef Foundation is an attempt to undermine a decision to help the reef, the prime minister says.
Malcolm Turnbull continues to rigorously defend making the record grant to the foundation, which the charity has admitted it did not apply for, suggest, or expect.
He particularly took aim at NSW Labor senator Kristina Keneally.
"She is trying to smear out what is a very responsible decision, taken, as I said, by the budget process through the cabinet system," Mr Turnbull told reporters in Perth on Friday.
Senator Keneally has dissected the grant since its announcement in April, which along with calls by the Greens led to a recent Senate inquiry to run a fine-tooth comb over the decision.
The inquiry revealed the money was offered to the charity in a private meeting organised by Mr Turnbull and Environment Minister Josh Frydenberg, without the foundation explaining how it would be spent.
The government maintains the foundation was chosen because of its proven ability to liaise with the private sector for funding, yet Labor says the charity has lied about raising $90 million in private funds since 2000.
Labor is calling for the money to be returned, as Mr Turnbull and Mr Frydenberg point to its ability to help the reef.
"The decision ... is a very appropriate and thorough process and it is a great grant which will go a long way towards maintaining the resilience of the Great Barrier Reef," the prime minister said.
The auditor-general has listed the grant as a "potential" inquiry in its 2018/19 work program but has yet to commit to a full audit.