Namah already denied recent Fairfax media reports that he sexually harassed a male black jack dealer at the Sydney casino last year and threatened to take legal action.
The reports also suggested Namah and two of his colleagues had $A800,000 in a gambling account.
"I call on relevant authorities like the Ombudsman Commission to tell PNG if Mr Namah has declared his various overseas incomes and bank accounts," Kumbakor said in a statement.
"Otherwise Mr Namah has to inform the public how he took such a large amount of money to Australia and from where (PNG or Malaysia?).
"His lawyer Greg Sheppard has already confirmed in the Australian media that the $A800,000 is Mr Namah's private money and not public funds."
Kumbakor - Sir Michael Somare's man for the defence portfolio if ever he becomes Prime Minister again - called on the Internal Revenue Commission to work with the OC to investigate Namah's income tax statements.
"Mr Namah has serious allegations still outstanding about money laundering," Kumbakor said.
"It was not too long ago that the story broke about the controversial flight on the Falcon jet from Malaysia to PNG where purportedly there were millions of US dollars on board.
"Even Samoa was investigating a company linked with Mr Namah and money laundering.
"These claims have not been cleared yet.
"Among others in search for their money, Bewani landowners are asking about their share in the Bewani palm oil development project.
"Our own leaders are bringing disrepute to our people and government and shaming us in the eyes of the world.
"The people of Papua New Guinea should not be tolerating these questionable incidents.
"Furthermore, not many Papua New Guineans I know can afford to gamble with that much money. I am sure ordinary businesspersons in PNG do not have such readily available disposable income."
Kumbakor said it was shocking that a member of parliament could gamble with "that kind of money" given the poor health and education indicators in PNG.
A variety of cases over the legitimacy of the O'Neill-Namah government remain before the Supreme Court.
Namah, who is also forestry minister, is one of the richest men in PNG's Parliament and is believed to have made a fortune in the forestry industry before entering politics.