The opposition leader, Bill Shorten, says the prime minister needs to be open about what happened at a meeting that led to the awarding of a $443.8m grant to a small not-for-profit foundation for the Great Barrier Reef.

“This is a developing scandal and if Mr Turnbull thinks it will just go away, he is mistaken,” Shorten said.

“Most Australians would be shocked to learn this prime minister personally just walks into a meeting and hands over nearly half a billion dollars of taxpayers’ money without a moment’s thought.

There is growing pressure on the government to reveal what took place at a 9 April meeting between Malcolm Turnbull, the environment and energy minister, Josh Frydenberg, and John Schubert, the chairman of the Great Barrier Reef Foundation, which was awarded the grant without a tender process.

On Friday, Turnbull defended the budget allocation, saying the government had acted transparently.

Speaking at a press conference after a book launch in Sydney, the prime minister said the Great Barrier Reef Foundation was “an outstanding organisation” and the government was “making the single biggest contribution and investment in the health of the Great Barrier Reef ever”.

“This has been done completely transparently. It was in the budget. It was actually announced before the budget,” he said.

“It’s been considered by the parliament and it’s been passed in legislation, so it’s gone through the parliament.”

He added there was an almost 100-page agreement between the government and foundation as to how the money would be invested and spent and it would be subject to oversight from the government and the Australian National Audit Office.

“So this is a wonderful investment in ensuring we maintain the health of the Great Barrier Reef,” he said.

There was no separate legislation for the foundation grant, it passed as part of the appropriation bills for the federal budget.

“What does he mean when he says it was considered by parliament and passed by legislation? There was no reef foundation bill put through parliament this year,” said Greens senator Peter Whish-Wilson, who is chairing an inquiry into the grant.

He said the committee had been liaising with the foundation for six weeks seeking to have several witnesses appear.

Whish-Wilson said due to the ongoing unavailability of some witnesses, it was unlikely further hearings would be held this month.

“We have asked the Great Barrier Reef Foundation to respond to the committee by next Wednesday with the earliest possible dates these witnesses can appear. It’s been disappointing that we haven’t been able to get a date sooner considering the intense public interest,” he said.

Schubert and fellow board members Grant King and Paul Greenfield were not present at Monday’s hearings.

A spokesperson for the foundation said Schubert had indicated that “to ensure full transparency in relation to the process, he and other board members will be available to appear at a second hearing”.

“The committee has asked us to come back to them by next Wednesday 8 August to advise available dates and we will do that,” she said.

Frydenberg appeared on the ABC’s 7.30 on Thursday night and said the environment and energy department had met more than 20 times with the foundation since early April and the federal and Queensland governments would be consulted on investment decisions.

“There’s a very public partnership agreement,” he said.

But Labor’s environment spokesman, Tony Burke, said the minister neglected to say that none of these meetings took place before “the largest donation of taxpayers’ money to a private foundation in history” was offered by the government.

“The fact that the department was then forced to rush in with so many meetings after the prime minister had already offered almost half a billion dollars to the Foundation doesn’t detract from the fact that this cash splash was done without process, probity or foundation,” he said.