Two-thirds of voters believe Helen Clark should have taken responsibility for her speeding motorcade.

The latest Herald-DigiPoll survey shows that 65.2 per cent of voters believe the Prime Minister should have stepped forward over the speeding, which resulted in Constables Ian Howard and Simon Vincent and a civilian driver, who cannot be named, being fined $675 each for dangerous driving between Waimate and Christchurch Airport last year.

The motorcade travelled at up to 170km/h to ensure she caught a flight to Wellington for a rugby test.

The survey shows more than half of Labour supporters - 50.9 per cent - think Helen Clark should have taken responsibility and 83 per cent of National supporters think so.

National Party leader Don Brash yesterday told the Weekend Herald the motorcade case had cost about a million dollars.

Deputy leader Gerry Brownlee said the $1 million figure came from costings National had worked out - taking into account such things as police time, court time and legal fees.

The Crown Law Office and the Office of the Police Commissioner said they were still working out how much the case had cost.

Dr Brash said there was a specific exemption in law allowing ministers to speed on urgent matters of state. He said there would have been no case if Helen Clark had said it was important she get to the rugby game.

"There is not much doubt that those police officers and the civilian driver took the rap for a situation which Helen Clark should have been responsible for."

Helen Clark would not comment on the survey. To date she has said little about the speeding incident, except that the police "were in charge of a motorcade".

On Monday, she told Paul Holmes on Newstalk ZB that she "would not have asked for urgent travel to go to rugby".

"They [the police] advised my press secretary what plane could be caught. I accepted that advice, I got in the car."

Cabinet ministers have decided to contribute $120 each to pay the fines and court costs of the three men. Public donations are also being sent to the Timaru police station.

Police Association president Greg O'Connor said about $3000 had been raised and any extra would go to the men so they could have a holiday with their families.