Australia’s environment department head has told an inquiry it is wrong to assume that no due diligence was undertaken before a record $444m grant was offered to a private foundation for Great Barrier Reef projects.

But the chair of a Senate inquiry examining the controversial grant to the Great Barrier Reef Foundation says he is “convinced now that there has been no long-term work into this proposal”.

His statement came after six hours of hearings on Friday again failed to establish who had the original idea to award the grant, and finance, environment and Treasury officials repeatedly refused to answer questions about the grant process, claiming cabinet and budget confidentiality.

In a statement to the Senate committee, the secretary of the Department of Environment and Energy, Finn Pratt, said “there is much interest in due diligence” undertaken for the grant, which was offered during a private meeting between the former prime minister Malcolm Turnbull, the then environment and energy minister Josh Frydenberg and the foundation’s chair, John Schubert.

“The former minister explained to the House the department did a first phase of due diligence as part of the cabinet process,” Pratt said.

“This included looking at the foundation’s constitution, structure, governance, board composition, project management, scientific expertise, fundraising history and capacity for growth.

“It is incorrect to assume that because the foundation was unaware of the department’s due diligence work that no due diligence took place before the offer to the foundation was made.”

The Greens senator and committee chair, Peter Whish-Wilson, asked environment officials if they could “put me out of my misery” and explain where the idea for the grant originated.

“Chair, I apologise if I continue your misery, but … this ultimately was a decision by the government in the budget process,” Pratt said. “I think the former minister’s statement in the House is about as close as we can go to elaborating on this.”

Frydenberg told the parliament in August that the government had undertaken a two-stage due diligence process.

“I read that statement a couple of times, Mr Pratt, and felt like he said it was a department decision,” Whish-Wilson said.

Pratt responded: “Well, can I reassure you that the department does not make decisions, the government makes decisions.”

Speaking to Guardian Australia after the inquiry, Whish-Wilson said the committee was drafting questions that would be sent to Turnbull, who is now in New York.

“It’s been a political decision at a very high level,” he said. “I’m totally convinced of that now, as I am of there not having been a long gestation period.

“We are in the process of writing to the former prime minister in New York. They will be initial questions, there may be others to follow.”

Depending on the responses, he said, Turnbull could still be asked to appear before the committee.

“There’s still some big unanswered questions about whose idea it was and when it was proposed.”