A former leading law official Papua New Guinea, Sir Arnold Amet, says Belden Namah will be required to give up the West Sepik governorship once an election is held for the regional seat.
Sir Arnold was speaking after Mr Namah, who was elected by local level leaders two weeks ago, announced his intention to retain the post after the by-election for the seat is held.
He is adamant he can retain the post despite the election of a new regional member in July.
But Sir Arnold says that is not the case.
"The law is strictly that when a by-election is held and the provincial member is elected, who is then automatically, ex officio, the provincial governor, he shall assume office. An acting governor in the interim, as has happened with Belden Namah's election, is only in the interim. When constitutionally the provincial seat has been substantively filled he is automatically, ex officio, the governor."