Last month the opposition gained a majority and adjourned parliament to December 1 as they prepared for a vote of no-confidence against Marape, reports Radio New Zealand International.
But within days the Speaker, Job Pomat, recalled parliament after ruling that the decision by his deputy, Koni Iguan, to entertain the opposition's adjournment motion was out of order.
The government quickly passed the 2021 budget, before promptly adjourning the house until April. The opposition, which was not present, filed a challenge in the Supreme Court against the sitting.
A five man-bench headed by chief Justice Sir Gibbs Salika has heard arguments from both opposition and government.
On December 9 the court has ruled that the opposition's adjournment of parliament last month was constitutional.
The bench found that the Speaker did not have the authority to overrule the decisions of the deputy Speaker and recall parliament, suggesting that Pomat's actions ran the risk of being in contempt of parliament.
Furthermore, the court ruled that parliament's sitting on November 17, when only government MPs were present, was unconstitutional.
This renders all parliamentary business conducted that day as invalid, including the passing of the national budget.
It advances the political crisis which developed last month into a critical phase, with lobbying among MPs intensifying ahead of a likely challenge to Marape's leadership.
PNG's opposition leader Belden Namah has said that his group still has a majority, although since last month's parliament proceedings, the numbers on both sides has evened up.
The opposition has not yet revealed who its candidate for alternative prime minister would be, although Namah's group also includes former deputy prime minster Sam Basil who led a mass defection from the coalition government last month.
PNG's former prime minister Peter O'Neill, who was replaced by Marape in May last year, was the applicant in the Supreme Court case and is also a key player in efforts to remove his former close ally.