The golden run of cheaper petrol could be at an end with prices at the pump increasing for the first time in four months, AA says.

All fuels have increased by 4 cents per litre, which AA PetrolWatch spokesman Mark Stockdale said was "a bit on the nose".

The blame could be placed on the low New Zealand dollar, which had fallen 5c in the past fortnight to the lowest level since September 2010, he said in a statement on the company's website.

"Commodity prices have also risen for the first time since August, which suggests the long run of reductions are over."

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Margins had now fallen to levels last seen in April last year, which meant fuel companies had passed on all of the commodity price reductions, Mr Stockton said.

"Now the imported cost of fuel has risen, but it would have been preferable if fuel companies raised prices only 2c at a time -- the same rate at which they cut prices -- a 4c hike in one go is a bit on the nose."

After a run of 22 straight price cuts in a row since October, petrol prices had fallen to their lowest levels since May 2010, and diesel to prices not seen since 2009, Mr Stockton said.

Z Energy spokeswoman Sheena Thomas said barrel prices had been "trending up" over the past couple of weeks and the company had held off increasing prices as long as possible

"So when the barrel was coming down we basically thought that it would be better to pass the savings on quickly, which meant we had to do it in smaller amounts rather than holding the price and cutting it by 4c all at once."

Fuel prices were volatile and it was impossible to know if low fuel prices had come to an end, Ms Thomas said.

BP and Gull were not immediately available for comment.

Petrol and diesel prices today:
Auckland

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91 Octane - 176.9

95 Octane - 185.9

Diesel - 110.9.

Wellington

91 Octane - 176.9

95 Octane - 185.9

Diesel - 110.9

Christchurch

91 Octane - 176.9

95 Octane - 185.9

Diesel - 110.9

Prices as of January 14:
Auckland:

91 Octane - 172.9

95 Octane - 181.9

Diesel - 106.9

Wellington

91 Octane - 172.9

95 Octane - 181.9

Diesel - 106.9

Christchurch

91 Octane - 172.9

95 Octane - 181.9

Diesel - 106.9

(All prices in cents per litre. Source: AA)