Papua New Guinea's Institute of National Affairs says it's uncertain whether parliament would have the power to determine who is the Prime Minister if the Supreme Court rules that the Peter O'Neill government is illegitimate.
Mr O'Neill came to power in August after the then leader, Sir Michael Somare, had been out of the country for five months, receiving medical treatment in Singapore.
The prime ministership was declarared vacant and Mr O'Neill installed with the backing of a large majority of the MPs.
The Institute's Paul Barker, says the Court will tomorrow rule on if there was a vacancy and MPs had followed a legal process.
"The next question, if it was declared that it was not legal, whether they would then just say hey it's now back to you, the parliament, to determine what the situation is. The Supreme Court is being put in a very difficult position - [but their] responsibility is there to protect the constitution."