Myanmar's junta sentenced ousted leader Aung San Suu Kyi to another three years in prison while also giving an Australian former government advisor the same term, a source told AFP on Thursday.

Both have been detained since a coup in February last year when the military ousted Suu Kyi's government, for which Sean Turnell, an economist, was an advisor.

"Mr Sean Turnell, Daw Aung San Suu Kyi and another three were sentenced to three years imprisonment each under the Official Secrets Act," the source told AFP, adding that Suu Kyi would appeal her verdict.

Suu Kyi has already been convicted of corruption and a clutch of other charges by a closed court, while Turnell is facing another case under the country's immigration act, which according to the source is ongoing.

Turnell's arrest last year prompted international outrage, with Australian diplomats lobbying Myanmar's regional neighbours to assist with the case.

The 76-year-old was in the middle of a phone interview with the BBC when he was detained after the 2021 coup.

"I've just been detained at the moment, and perhaps charged with something, I don't know what that would be, could be anything at all of course," Turnell told the broadcaster at the time.

In August, he pleaded not guilty to breaching the colonial-era official secrets act during his trial in a secretive junta court – inaccessible to journalists – in the capital Naypyidaw.

He was facing a maximum penalty of 14 years in prison.

The exact details of Turnell's alleged offence have not been made public, though state television has said he had access to "secret state financial information" and had tried to flee the country.

Myanmar has been in turmoil since the military seized power last year and ousted Suu Kyi's elected government. (AFP)