The missing piece of evidence Australia is still pursuing after Zomi Frankcom's killing

The family of slain Australian aid worker Zomi Frankcom say they will continue their fight for justice, with missing drone audio remaining one of the central unanswered questions about her death.

Israel this week announced it would not prosecute those responsible for the killing of Frankcom and six of her World Central Kitchen colleagues in Gaza in April 2024, despite conceding "serious failures" that led to the mistaken-identity drone strikes by the Israel Defense Force (IDF) .

On Thursday, Foreign Minister Penny Wong said the decision was an "outrage" , and she had no confidence in the IDF's Military Advocate General's two-year investigation "given the many unanswered questions".

Among the unresolved issues is a missing piece of drone footage audio, which Israel refused to hand over to Australia's independent inquiry, headed by retired Air Chief Marshal Mark Binskin, despite requests by the federal government.

According to his report — which found the killing was due to mistaken identity and procedural failings but not a deliberate targeting of aid workers — Binskin was given access to 90 minutes of unedited drone footage, but it did not include an audio track.

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Prime Minister Anthony Albanese renewed calls for the audio to be released in parliamentary Question Time on Thursday, saying it's part of the transparency Australia has called for "from day one".

Israeli ambassador to Australia Hillel Newman on Friday told ABC radio the audio may contain "intelligence information" and there was nothing in it to cast doubt on the findings which cleared the IDF of criminal misconduct.

In a statement on Thursday, Frankcom's family called on the government to continue pressing Israel to release the audio and to "pursue all available legal avenues for an independent and credible investigation to be conducted".

However Dr Emily Crawford, an international law professor at the University of Sydney, said Australia's options were limited.

"In all states, the scope to argue that national security grounds required for information not to be released, they're fairly paramount," she told SBS News.

"There's not anything they could really do beyond kind of diplomatic measures.

"We've seen that the Netanyahu government is reluctant to assist in these kinds of things, so I can't imagine there's really going to be any movement on that."

The missing audio is just one of the unanswered questions surrounding the strike, with Wong saying the IDF had failed to explain how two strikes occurred after the initial strike, in breach of the IDF's rules of engagement.