Former prime minister Malcolm Turnbull has blamed the rush to grant almost half a billion dollars to the Great Barrier Reef Foundation in one hit on his successor, Scott Morrison and Finance Minister Mathias Cormann.

In a written response to a Senate inquiry into the controversial $443.4 million outlay to the tiny non-profit group, the ex-PM said Mr Morrison, the then treasurer, and Senator Cormann "were open to funding a substantial package for the Reef so long as it was expensed in 2017/18". (Italics in the original.)

The powerful Expenditure Review Committee of Cabinet met on March 6 this year to consider a budget submission by then environment and energy minister Josh Frydenberg, who proposed two options to boost the health of the Great Barrier Reef, Mr Turnbull said.

The choice was between a six-year proposal "partially offset" by savings elsewhere and a smaller, second proposal "fully offset over two years".  Instead, the committee picked an alternative proposal creating a tied reef fund with a partner outside the government sector.

The purpose of bringing forward the spending in one go in that fiscal year "was because government revenues were promisingly strong in 2017/18 and [the Treasurer and Finance Minister] believed the budget in that year could accommodate the substantial investment proposed for the Reef", Mr Turnbull wrote. "[T]hat may not be so in subsequent years."