The Prime Minister is refusing to take responsibility for a speeding motorcade which rushed her to catch a plane on Saturday night, blaming police for the decision to speed.

But One News has found that Helen Clark could have caught a later plane and still made it to the Bledisloe Cup test she was rushing to attend.

Clark was racing to catch the 5:50pm flight from Christchurch to Wellington.

One News found there were nine seats free on a later flight that would still have left the Prime Minister with an hour to make the 7:30 kick-off.

Clark says she was working in the back of the car and did not notice the speed the cars were travelling.

Breaking the speed limit is allowed if a minister is on urgent public business.

Clark will not be drawn on whether her weekend dash meets those criteria.

"That's not a judgement that I made. The police made a judgement about the mode of transport and the way it was done and they will have to deal with these issues when they do their investigation.

Transport authorities spend $2.5 million dollars a year advertising speed as the number one killer.

Speed is a factor in one in three road deaths on New Zealand roads.

Meanwhile, opposition parties are demanding the Prime Minister take more responsibility for the high speed police motorcade she was in on Saturday.

National's Law and Order spokesman Tony Ryall says she must have known how fast the car was going while ACT says it appears the Prime Minister is trying to put the blame on the police.