US president-elect Donald Trump's recently-appointed national security adviser was investigated for inappropriately sharing highly-classified intelligence with Australian forces.

Retired US three-star lieutenant general Michael Flynn, a maverick who spent more than 33 years in US Army intelligence, worked alongside Australian forces in Afghanistan and Iraq.

An outspoken believer in assisting allies on the battleground despite red tape preventing the flow of information, Lt Gen Flynn said the sharing of intelligence with Australian and British forces that left him in hot water was done "with the right permissions".

"I'm proud of that one," Lt Gen Flynn told the Washington Post. "Accuse me of sharing intelligence in combat with our closest allies, please."

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His unconventional style and strong resume - he was tapped by US commander in Afghanistan General Stanley McChrystal to be his top intelligence officer and promoted by President Barack Obama as Defence Intelligence Agency director - was obviously attractive to fellow maverick Trump.

Lt Gen Flynn was pushed out of the DIA job after two years in the role and has labelled Obama a "liar".

His views on the Middle-East are aligned with Trump and both men are prolific users of Twitter.

During the recent presidential campaign Lt Gen Flynn, a registered Democrat, called Trump's chief opponent, Democrat Hillary Clinton, "the enemy camp" and joined the call to "lock her up" in jail.

He also raised eyebrows when he sat alongside Russian President Vladimir Putin at a lavish party in Moscow last year.

Retired Lt. Gen Michael Flynn has an unconventional style. Photo / AP
Retired Lt. Gen Michael Flynn has an unconventional style. Photo / AP

Offering insight into his more open, untraditional philosophy of sharing information, in 2010 he co-wrote the report Fixing Intel: A Blueprint for Making Intelligence Relevant in Afghanistan.

It concluded the US intelligence community "must open their doors to anyone who is willing to exchange information, including Afghans and NGOs (non-governmental organisations) as well as the US military and its allies".

Lt Gen Flynn has confidently defended the incident that involved passing sensitive information to Australia and Britain.

"The investigation on me was for sharing intelligence with the Brits and Australians in combat, and I'm proud of that one," Lt Gen Flynn said. "That was substantiated because actually I did it.

"But I did it with the right permissions when you dig into the investigation." Lt Gen Flynn said he met with Trump mid-2015 and described the real estate billionaire as a "very serious guy", "good listener" and possessing similar views.

"I found him to be in line with what I believed," he told the Washington Post.