Helen Clark told the House her advice was that the UN Charter authorised military action against alleged mastermind Osama bin Laden and his al Qaeda network on the grounds of self-defence.
"We would support such action on that basis."
She also described the supporting Security Council resolutions as "very strong".
In an effort to satisfy Alliance concerns, Helen Clark sought to amend Wednesday's parliamentary motion backing the offer of SAS troops, to ensure it included the two Security Council resolutions.
But Mr Robson said yesterday that neither of the two resolutions authorised the unleashing of military force.
"It is the Alliance's opinion that the use of military force in the present situation will require the explicit and previous authorisation of the Security Council," he told Otago University students.
"There is no need to take precipitate action that short-circuits international law.
"The Alliance does not believe that a massive military attack on a disintegrating nation will deliver the result the world is seeking. It is time for cool heads and patience."
He said that under the UN charter, the right to individual or collective self-defence allowed repulsion of "immediate attack" before handing the issue over to the Security Council.
"The Alliance has a long history of insisting on observance of international law and the authorisation of the UN for military action."
Greens' co-leader Rod Donald, whose party voted against the parliamentary motion, was pleased that Mr Robson was now contradicting the Alliance's support for the motion.
"Their supporters need to know one way or the other where the Alliance stands."
Map: Opposing forces in the war against terror
Afghanistan facts and links
Full coverage: Terror in America
