The Department of Defence has refused to answer questions or explain its about-face, despite concerns the organisation misled Senators.

However, the public has been urged to "always assume a screw up before a conspiracy".

In October, the Canberra Times reported Australia ignored the United States-led Five Eyes meeting about Unidentified Anomalous Phenomenon (UAP) - the contemporary term for UFOs - despite the US labelling the issue a "national security threat".

The Department of Defence denied attending multiple times, including in a response to a question on notice by Greens Senator Peter Whish-Wilson in the May Senate estimates.

In the following November estimates, Senator Whish-Wilson continued to push the department to explain itself, questioning why Australia would turn down a briefing invitation to its most important intelligence alliance and how many other Five Eyes meetings it had not attended.

"For what reasons did the Australian Department of Defence decide that Australia - as a Five Eyes member - not attend AARO's Five Eyes Forum on UAP?" Senator Peter Whish-Wilson said.

In response, the Department of Defence backflipped on its previous answer.

"Yes [Australia was invited to the UAP briefing]. A Defence representative at the Australian Embassy in Washington attended," the department stated.

It's understood an Australian Defence attache based in the US attended the briefing, which was held in May 2023.

However, the Department of Defence would not confirm the details and refused to answer any questions about why it had misled Senators in its previous response.

Ross Coulthart is an Australian journalist and one of the world's leading UAP investigators.

He said it was quite plausible the Department of Defence didn't do its homework before brashly responding to the Senator's inquiry.

"The first explanation is just incompetence, and they treated the inquiry with such disrespect they didn't properly check with their Defence attaches in Washington," Mr Coulthart said.

"The worst explanation, the one that I don't like to think about, is that they were knowingly lying, and I sincerely hope they weren't.

"Always assume a screw up before a conspiracy. The bottom line is that they should have been more clear up front in the first place.

"Maybe it was just incompetence and they hadn't asked all the right questions. But it demonstrates they treat this subject with a flippancy and disregard that their American ally doesn't."

Grant Lavac, an Australian civilian UAP researcher, said Defence's silence on the issue was concerning and called on the government to explain itself.

"What confidence can Senators have that their questions will be accurately answered, when the Department of Defence cannot even offer an explanation as to why it denied Australia's participation in the Five Eyes forum, only to then make a complete backflip admission?" Mr Lavac said.

"It does not bode well for the government and the Department of Defence's commitment to transparency, when accredited members of the news media and private citizens cannot even solicit comment and a simple explanation."

The US-led Five Eyes alliance consists of the United Kingdom, Australia, Canada and New Zealand. The Canadian and New Zealand governments confirmed they had defence personnel attend the UAP forum.

Australia's UAP policy has previously been criticised for being out of step with its closest military allies, including the US, which introduced mandatory UAP reporting for defence personnel in 2021.

The past three years, the US has held multiple Congressional hearings and commissioned several reports, while the Pentagon has created a new division dedicated to researching UAPs.

Other allies have acknowledged the unknown aerial phenomenon, including the UK, Canada, France and Spain.

The Canberra Times has previously reported Australian Defence personnel don't feel comfortable reporting UAP sightings through official channels and freedom of information documents revealed senior Defence officials mocked the subject while preparing briefing notes.

The Australian Defence Force stopped collecting reports of UAPs in 1996, "after determining there was no scientific or other compelling reason to continue to devote resources to the recording and investigation of UAP".

Despite the renewed interest in the topic from other Western nations, Australia has no plans to implement reporting mechanisms for pilots.