Papua New Guinea will become like strife-torn Zimbabwe if the prime minister continues to intimidate and threaten the nation's watchdogs, the PNG opposition says.

Prime Minister Michael Somare on Thursday used parliamentary privilege to attack recently retired Ombudsman Commission chief Ila Geno for initiating an inquiry into his personal finances.

Somare is facing a probe into alleged financial irregularities, some dating back 20 years.

He lost a court bid last month to stop the inquiry from proceeding, but has launched an appeal in the Supreme Court.

Government MPs rallied behind Somare in Thursday's heated parliamentary session, saying he had "served PNG for 40 years" and was a "big man" beyond reproach, especially by the Ombudsman's office.