Petrol prices have gone up by an extra three cents a litre today following an increase in imported petrol prices.

Today, 91 octane in Tauranga is $2.20 a litre and diesel is sitting around $1.54 at most petrol stations.

Pricewatch.co.nz lists 91 octane at Caltex at $2.14, six cents cheaper than opposition Z Energy.

The rise in petrol prices is being attributed to increasing input costs, with Brent Crude trading at a nine-month high of US$118.65 a barrel.

AA PetrolWatch spokesperson Mark Stockdale says while the NZ exchange rate remains strong, commodity prices have continued to rise in the last week.

He says the imported price of petrol rose two cents since the last increase on January 31, while diesel costs are up five cents since prices fell on January 18.

'At current retail prices, the importer margin (retail price less costs) for both fuels has been restored to upper end of the normal range (whereas after last week's increase, petrol margins were at the lower end).”

AA PetrolWatch says there are two main reasons for rising fuel prices.

The first is that world demand for fuel is exceeding exploration and refinery capacity. This is what has traditionally caused prices to rise.

When supply is tight, disruptions or the mere threat of disruption to oil well or refinery capacity – such as war or a natural disaster – can cause prices to spike, says the organisations website.

The second reason is investment speculation. Money from investment funds is being invested in commodities like food and oil in response to a weaker world economic climate and a weak US dollar. This investment in oil stocks is helping to drive up prices.

'Increases in world oil or refined product prices result in fuel price rises at the pump, though if the NZ dollar is strong against the US dollar motorists are buffered to an extent. When the NZ dollar falls, pump prices rise because all fuel is purchased in US dollars.

'Generally, any international price changes are almost immediately passed on to the motorist. This works well for consumers when prices are falling and when there are both price rises and falls, but is often seen as unfair when prices are continually rising. Motorists quite rightly feel they are paying today's new price for product that was purchased by the oil companies at yesterday's lower price,” reports the AA PetrolWatch website.