By EOIN BLACKWELL of AAP
"Papua New Guinea is fast becoming a banana republic," Mr Namah said.
"We say, with both anger and sadness, that the O'Neill-Dion government is a serious national threat to the independence, sovereignty and security of our country."
The law, which will extend the so-called "grace period" on votes of no-confidence from 18 months to 30 months after a government is elected, passed the first of two votes in November.
At the time, the opposition voted in favour of the law.
But Mr Namah says scandals - including the granting of citizenship to Indonesian fugitive Djoko Tjandra last year - is sending the wrong message internationally.
"Is the government sending a message to transnational criminals that PNG is a haven for them as long as it is in government?" he said.
The government on Wednesday night moved to revoke Djoko's citizenship.
Mr Namah also accused Indonesian security forces of making frequent incursions across the West Papuan border into PNG.
The extension of the grace period, aimed at stabilising PNG's often rocky politics, will become law if it passes a second vote scheduled for February 5.