The governor of Papua New Guinea's Enga Province, Sir Peter Ipatas, is the latest senior MP to defect to the opposition.
The group plans to move a motion of no confidence to topple Prime Minister Peter O'Neill, which could happen as soon as tomorrow.
Dozens of MPs have set up camp at a Port Moresby hotel ahead of the vote and James Marape has been nominated as their alternative prime minister.
Both the government and the opposition claim they have the numbers to form a government.
Arriving at the opposition's Laguna Hotel, Sir Peter said his patience had run thin with Mr O'Neill.
"I have warned this government, and I have been talking to the Prime Minister [for] many years about allowing the processes and systems to work. It has fallen on deaf ears and I believe it is not in the best interests of our country.
"I know it is not in the best interest of my people or my province that I represent in parliament. We must always be mindful that the people's interest is paramount."
Meanwhile, Papua New Guinea's finance minister has reaffirmed his support for Prime Minister, Peter O'Neill.
Sam Basil's Pangu Pati split last week after six MPs defected to the opposition.
The remaining nine MPs say they have joined the Melanesian Alliance.
Mr Basil, who last year was the deputy leader of the opposition, said the nine MPs will support Mr O'Neill.
"We are inside the coalition government camp, and when we are here you already know the answer - we will support."
Sam Basil said it's important that governments last between elections.
Commissioner urges MPs to respect neutrality of police
Papua New Guinea's police commissioner is urging Members of Parliament to leave the police out of their squabbles.
In a statement, Gari Baki appealed for MPs to respect the neutrality of the police, and to not use officers for their political agenda.
He also urged officers not be influenced by MPs.
He said police should not be used to guard MPs' camps, or to be their escorts.
More than 1000 police have been deployed around the capital, Port Moresby, in anticipation of the vote of no confidence against the prime minister.