Malcolm Turnbull is unsure why a small not-for-profit organisation was chosen for a controversial $444 million government grant without a tender process.

The former prime minister has revealed his version of events behind the Great Barrier Reef Foundation being given the money during one brief meeting in April.

Mr Turnbull's response to questions from Greens senator Peter Whish-Wilson, who is looking into the funding allocation, has been made public.

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He had been asked to appear before a Senate inquiry, which is due to hand down its report on Tuesday.

But with Mr Turnbull holidaying in the United States after quitting politics, he wrote to Senator Whish-Wilson instead.

Mr Turnbull was unable to give a firm answer about why the foundation was singled out for the grant.

"As to why the GBRF was recommended by the Department of the Environment and Energy (as opposed to another organisation) you should inquire of the department," Mr Turnbull said.

"But it was plainly a reputable organisation with a track record in reef research and support with whom the department had previously worked (with)."

www.barrierreef.org

Mr Turnbull did confirm Prime Minister Scott Morrison's role in the money being handed over in one financial year, rather than spread it out in various budgets.

Mr Morrison told reporters on October 1 - the same day Senator Whish-Wilson received Mr Turnbull's letter - that it was a "pretty good common sense" decision he took when he was treasurer.

Senator Whish-Wilson said while it confirmed Mr Morrison's and Finance Minister Mathias Cormann's role in the saga, there were still questions left unanswered.

"We are still none the wiser as to whose idea it was to give the money to this small private foundation in the first place," the senator said on Friday.

"It seems like no-one wants to take responsibility for this."

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Mr Turnbull's evidence came on the same day the foundation launched a campaign to raise a further $400 million.

The foundation declared on Friday it wanted to raise between $300 million to $400 million from private donors over six years.

Mr Turnbull said in his letter there had always been a possibility the foundation could leverage the government funds with the private sector.