Paita, the finance minister, was among several ministers to defect in a group of 18 from Marape's Pangu Party.
Newspaper front pages in the capital Port Moresby have been dominated by the political drama, which has overshadowed a devastating landslide estimated to have buried some 2000 people in remote villages in Enga province on Friday, where rescue efforts are still underway.
Parliament sits at 2pm on Tuesday, returning after three months, with a motion of no-confidence expected to be lodged, officials said.
Before the motion can be voted on, it needs to be approved by a parliament committee that sits on Wednesday, the parliament speaker's office added.
In a statement on Monday evening, Marape said six of the 18 lawmakers who defected had now returned to the government side, and his coalition partners had also pledged support.
The parliamentary majority threshold is 56.
"We had a very big coalition of 93 MPs, which has gone down to just over 70," said Marape.
He said his coalition partners had agreed "now is not the time for instability but reconstruction under the present government".
Marape was re-elected to a second term as prime minister in 2022, and has already survived a no-confidence vote in February after riots in the capital.
"Tomorrow, the government will have the numbers but the consensus is that we as a government must work hard to deal with the issues affecting our country genuinely and seriously," Defence Minister Billy Joseph wrote in a Facebook post on Monday evening.
