Scott Morrison has sidestepped a question about whether changes to a spreadsheet of sports grants on 10 and 11 April last year – including a change sought by his own office – were made with proper legal authority.

The former sports minister Bridget McKenzie reignited the sports rorts controversy late last week by insisting she made no changes to the brief and attachments outlining successful projects funded under the sports grants scheme after 4 April 2019.

The Australian National Audit Office told Senate estimates last week the prime minister’s office had requested a last-minute change before the documents were sent to Sports Australia at 8.46am on 11 April, which was the day parliament was prorogued for the 2019 election.

But another set of changes were made later that day, and the material was resent to Sport Australia at 12.35pm and 12.43pm.

“For example, when I referred to one project coming out and one project coming in, in terms of the 8.46am version, that was at the request of the prime minister’s office,” Brian Boyd, the ANAO’s performance audit services group executive director, told Senate estimates last Monday night.

“But of the changes made later that day, for the 12.43pm version, none were evident as being at the request of the prime minister’s office rather than the minister’s office making the changes.”

In her statement issued last Thursday night after parliament rose, McKenzie suggested that any changes made to the material after 4 April were done without her knowledge.

But she accepted ultimate responsibility for the decisions. “I was the minister for sport and therefore ultimately and entirely responsible for funding decisions that were signed off under my name, including and regrettably, any changes that were made unbeknown to me.”

Morrison was asked on Wednesday who made the changes to the spreadsheets after 4 April, and on what legal authority, given ministerial staff cannot exercise the decision-making power of ministers.

The prime minister dead-batted the question, saying “ministerial authority for the program was with the minister for sport”.

As well as persistent criticisms that the sports grants were mismanaged, with funding being distributed to targeted seats rather than on merit, there are also significant legal questions hanging over the controversial program.

Respected legal academics have queried the constitutionality of the program on the basis the federal government lacks power to hand out money to sports clubs.

In addition to the constitutional questions, the Australian National Audit Office, which triggered the whole imbroglio with a scathing assessment of the way the program was managed, has raised a separate issue about whether McKenzie ever had the legal authority to be the decision-maker for the grants.

McKenzie’s statement last Thursday also raises a third issue, because the former sports minister says she made no changes to the brief or the attachments after April 4. The ANAO and Sports Australia have made it clear that changes were made after that time. It is unclear who made the changes, and on what legal authority.

While the government is trying to move on, a Senate committee continues to examine the program, with another hearing scheduled for Thursday.

On Tuesday, Geoffrey Lindell, emeritus professor in law at the University of Adelaide, told the inquiry he had “serious doubts” that McKenzie had authority to approve the grants, echoing concerns of the Australian National Audit Office and other experts including Prof Anne Twomey. He told the inquiry McKenzie could be personally liable for failing to seek advice about whether she had the legal authority to give out $100m of sports grants.

Lindell noted that Sport Australia had a power of delegation, but the minister was not listed as a possible delegate, and the minister had a power to set “practices and policies” but that did not allow her to supplant Sport Australia as the decision-maker.

Lindell noted several law firms have threatened to bring legal challenges, suggesting that if they succeeded, the minister’s decisions would have “no legal effect” and Sport Australia would have to decide the grants afresh.

Maurice Blackburn has written to Sport Australia on behalf of the Beechworth Lawn Tennis Club, an unsuccessful applicant, asking that it approve its grant application by 17 March or face legal action in the federal court.