Petrol is expected to rise to $3.50 a litre for 91 octane by Christmas.

High oil demand globally is a contributing factor - as well as a weak New Zealand dollar - trading for about $59.2 US cents.

AA principal motoring affairs policy advisor Terry Collins says prices for crude oil have steadily increased in the last few months.

"The scary part is, in the last few weeks, it's gone up 14.5 percent. And we haven't seen those prices come through yet- that $95 a barrel quoted? That's October's oil."

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