Prof Patman took part in an academic forum at the University of Otago yesterday called "After Chilcot: The Consequences of the Chilcot Report for Armed Intervention and War''.
It was convened by National Centre for Peace and Conflict Studies director Prof Kevin Clements.
Sir John Chilcot's report, which took seven years to produce and was released last month, concluded Britain joined the invasion without exhausting other options before taking military action.
It highlights in precise detail the shortcomings of the decision-making process.
Prof Patman said the report held an important lesson for all politicians to be sceptical of information, and not to cherry-pick information that suited their convictions.
Prof Patman said former prime minister Tony Blair claimed his decision to support the invasion was based on intelligence reports, yet more than 65% of the reports he received did not support the claim that Iraq posed a threat.
Prof Patman said the media had not come out of the inquiry well, as journalists had not been sufficiently sceptical leading up the ill-fated decision in 2003.
The war had a "horrendous'' financial and human cost and had destabilised the Middle East. As well as fuelling extremist groups, it made Iran more powerful in the region, Prof Patman said.
Prof Clements repeated his call for Mr Blair to be impeached by the British Parliament, saying there had to be sovereign accountability over the "disastrous'' decision.
