By Ilya Gridneff, AAP

Papua New Guinea Prime Minister Michael Somare has been found guilty of 13 charges of misconduct in office for submitting annual financial statements late or incomplete.

A three-member tribunal sitting in Port Moresby handed down its decision on Monday afternoon and will reconvene on Tuesday to hear recommendations for an appropriate penalty.

Somare, 74, who has been at the forefront of PNG politics for 40 years, will remain in the top job and is expected to be punished only with a small fine.

Tribunal chairman Roger Gyles told the packed courtroom Somare was guilty of eight charges relating to filing incomplete financial statements as is required under PNG's leadership code.

Somare was also found guilty on five charges related to providing late records, Mr Gyles said.

But Somare escaped the three more serious charges of failing to provide any statement at all.

The tribunal also dismissed nine charges as irrelevant from the original 25 against the PM that dated back as far as 20 years ago.

Monday's brief session, during which Mr Gyles read out the tribunal's findings, is the fifth day of a hearing that began on March 10 after months of controversy and political rumblings.

Since 2008, Somare had tried to block the tribunal through the courts and late last year his lawyers made an unsuccessful bid for the Supreme Court to stop the proceedings due to bias and procedural flaws.

Somare told the court last week he had lodged his returns every year and suggested staff might have lost some records that were missing from the Ombudsman Commission.

But the commissioner who initiated the original investigation argued that Somare had failed to lodge or did not lodge complete forms or did so late and thus breached his responsibility as prime minister.

Every day of the trial the packed courtroom has been a who's who of PNG's political elite, with at times the country's entire cabinet present to support the prime minister.