Petrol prices are climbing much faster in the South Island compared to the North Island.

Fuel prices in the South Island have risen on average more than 100 per cent more than North Island prices in the past month, figures show.

New Zealand-owned technology company Gaspy allows motorists to track how much petrol costs at competing petrol stations, using crowdsourced information to update prices in real time.

Figures supplied by Gaspy show that between April 14 and May 11 91 octane petrol prices in the South Island had risen on average 9.3 cents a litre compared to 2.4 cents a litre in the North Island.

SUPPLIED Gaspy co-founder Larry Green says it collects pricing information on every petrol station in the country.

Marlborough had the biggest increase of 11 cents per litre followed by Otago with 10.1 cents a litre and Nelson increasing 9.9 cents per litre.

READ MORE: Under-fire petrol industry raises prices to highest level since 2014

Meanwhile Auckland's average petrol price rose just 1.7 cents per litre. Gisborne prices rose 1.1 cents per litre and Taranaki experienced the lowest increase of 0.7 cents per litre.

The average price across the North Island on Friday was $2.06 compared to $2.20 in the South Island.

The West Coast had the most expensive fuel with 91 octane costing on average $2.23 while Taranaki had the cheapest at $2, Gaspy figures showed.

The petrol industry's pricing tactics have been under scrutiny after a leaked internal email sent by a BP manager recently revealed it was orchestrating price rises across its central North Island stations in an attempt to manipulate competitor pricing.

The revelation prompted Energy Minister Megan Woods to call the company's senior management into the Beehive for a meeting.

Since then New Zealand's major petrol companies raised prices to a fresh three-and-a-half year high in many parts of the country with Z Energy, Caltex and BP sites across Wellington all increasing the price of regular 91 octane petrol by 2 cents a litre by Monday morning, to $2.199 a litre.

Gaspy co-founder Larry Green said South Island motorists were getting a rough deal.

"They're an easy target and they're a long way from Auckland," Green said.

"It's like they're the poor kid getting hammered by the bully and no one is doing anything about it."

Fuel companies appeared to be rising prices ahead of the 11 cents per litre Auckland regional fuel tax which will come into effect on July 1, he said.

"Price spreading will be absolutely happening."

By subtly increasing prices in the lead up to Auckland getting its tax, motorists across the country were less likely to notice, he said.

"What they're trying to avoid is backlash from consumers."

Come July 1 fuel companies would probably spread Auckland's 11 cents per litre regional fuel tax across to other regions, he said.

"They will double hike for sure if they can get away with it."