Petrol and diesel prices are on the up - but not in Christchurch.

BP and Mobil have increased petrol and diesel by 5 cents a litre, Caltex has increased 91 octane by 6 cents a litre, and Shell will introduce its increase tonight.

However, reacting to last week's devastating 6.3 earthquake in Christchurch, Caltex has only raised prices in the North Island and BP, Mobil and Shell will not increase Christchurch's petrol prices.

The increase means, on average, 91 unleaded at BP Connect sites would cost 207.9c outside the quake-hit city.

AA PetrolWatch spokesman Mark Stockdale said it was a very commendable gesture from the oil companies to exclude Christchurch from the raise.

"I don't think the people of Christchurch need to worry about the price going up any time soon," he told NZPA.

Oil prices went up last week and it was inevitable there was going to be an increase, he said.

BP New Zealand managing director Mike McGuinness said the company had been forced to react to soaring international prices, and a fall in the New Zealand dollar against the Unites States dollar.

"We have made the decision to not change our prices in Christchurch because of the difficult situation there."

Similarly, Shell owner Greenstone Energy chief executive Mike Bennetts said holding prices in Christchurch was the right thing to do for now.

"Many other companies are considering how best to respond to the needs of the people in Christchurch.

"We feel this is an appropriate and direct response for us at this point in time in addition to the other contributions that we have been making."

He said there may be a time when prices had to move in Christchurch but the company would hold off for the foreseeable future.