Motorists are saving an average of $8 each time they fill up their car compared with just a month ago, after petrol prices were slashed again.

From a recent peak of $2.20 a litre in May, unleaded 91 petrol plummeted to $1.99 at the major national petrol station chains yesterday, with family-owned Gull undercutting them at $1.95.

The cuts came in response to Brent crude oil falling to a six-month low of US$92.33 a barrel.

AA Petrol Watch spokesman Mark Stockdale said further petrol price falls could be coming.

"The fuel price cuts are coming thick and fast. The latest decreases, while welcome, really reflect last week's commodity prices, and since then they've fallen again.

"If you do need to fill the car up, you should take advantage of that lower price but don't panic – if you want to wait till next week you may save even more money."

BP spokesman Jonty Mills said the factors that determined prices at the pumps were volatile.

"The drivers at the moment are trending downwards, which is good news for the customers. The main drivers are a combination of the price of refined product on the international market with the economic and political volatility in Europe particularly around Greece, Spain and Italy, coupled with the exchange rate, which has been creeping up a little bit."

While the fall in petrol prices is making running a car cheaper in the short term, petrol is expected to become more expensive in the future.

Green Party transport spokeswoman Julie Anne Genter said petrol prices would continue to fluctuate as the global economy struggled with higher energy prices.

The cost of petrol was expected to more than double in the next decade, having already doubled in the past decade. "The smart move for New Zealand is to reduce its dependency on oil now," she said.

- © Fairfax NZ News