The environment minister, Josh Frydenberg, has again refused to answer simple questions about the process that led to the Great Barrier Reef Foundation receiving a $444m government grant without a competitive tender.

The Labor party last week called for the government’s $443.8m grant to be handed back, saying it ought to be returned to taxpayers and managed with probity.

Labor and the Greens continue to pressure the government to explain whose idea it was to give the money to the foundation in April with apparently little oversight.

Frydenberg told the ABC’s Insiders program on Sunday that the idea to give money to the foundation was contained in a budget submission he brought to cabinet last year, which was prepared by his department, but he refused to clarify if it was his idea or not.

The host of Insiders, Barrie Cassidy, asked: “Who raised the possibility of putting the money into the foundation?”

Frydenberg: “It went through the budget [process].”

Cassidy: “Who raised the idea?”

Frydenberg: “It was my submission [to cabinet].”

Cassidy: “It was your idea?”

Frydenberg: “It was my submission as minister through the cabinet process.”

Frydenberg then said his department had told him “a large funding commitment” to the foundation would comply with the government’s grant guidelines, but he would not clarify if that advice had been given before or after the grant had been made to the foundation.

Cassidy asked him what steps he or Malcolm Turnbull had taken to satisfy themselves that the foundation was the best organisation to accept the grant.

Frydenberg did not answer the question directly, saying the department had made it “very clear” that the foundation was the best organisation that could use the money to leverage off the private sector.

Cassidy persisted: “What steps were taken to satisfy yourself that’s the case?”

Frydenberg replied: “My department made it very clear that this is the best ... ”

Cassidy: “What did you do, what did the prime minister do, to satisfy yourself there was nobody else out there that was better qualified than the foundation to take the money?”

Frydenberg: “This is the best foundation and the best organisation ... ”

Cassidy: “What did you do to satisfy yourself of that?”

Frydenberg: “There was extensive due diligence.”

Cassidy: “There was no tender called?”

Fryndenberg: “If you want to talk about tenders. The Labor party has provided billions of dollars without tender. It is not unusual to provide large grants.

“The Australian national audit office did a report in 2012 which found that, when the Labor party was in office between 2007 and 2010, they looked at nearly 400 of their grants and more than a third of those did not go to a competitive tender process.

“Let me make it very clear. The foundation was chosen because it is the best organisation to leverage off the private sector.

“My department has made it very clear that the guidelines were complied with, that it represented value for money and it was the best way to deal with the urgent need of putting money into the foundation.”

On Sunday, the Labor senator Kristina Keneally said the Senate inquiry into the grant to the foundation still had many questions to ask.

“The department may not have known the prime minister was going to make this offer [of $444m] and they have been now asked, since that offer was made, to go and negotiate,” she told Sky News.

“I do not doubt that the department, and there’s plenty of evidence to suggest that since 9 April, when the prime minister made that offer, there have been meetings and discussion between the foundation and the department, but what we can’t seem to ascertain is whether there was any discussion prior to 9 April.”

In May, when news broke that the Great Barrier Reef Foundation had been awarded a $444m grant from the federal government, Anna Marsden, the group’s chief executive, said it was like winning “lotto”.

“We didn’t have much time before the announcement to be prepared for it,” she was reported saying in Fairfax Media. “It’s like we’ve just won lotto – we’re getting calls from a lot of friends.”

The $443.8m grant – made under the government’s 2050 Reef Partnership Program – was awarded in the May budget but the entire thing was booked in full in the financial year ending in June.

It meant the money was handed to the foundation in a lump sum, which is unusual for grants of that size.

Frydenberg said on Sunday: “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 need as they meet their objective.”