Using a discount card shaved between 6c - 15c per litre off the price.
The cheapest fuel in Auckland according to Gaspy was $2.63 for a litre of 91 at newcomer Costco Westgate which opened just two weeks ago.
The price advertised is considerably cheaper than nearby Pak'nSave at $2.77.
The catch is the fuel at Cosco's self-service pumps is only available to Costco members.
An annual membership with the US giant costs $60 or $55 for business.
Premium petrol had climbed to $3.45 in parts of Auckland.
MTA energy and environment sector manager Ian Baggott told the Herald the bulk of the price drivers pay for petrol is made up of taxes.
Just over half of the cost (52 per cent) is made up of taxes, including the fuel excise duty, the emission trading scheme levy and GST.
"Another 37 per cent is made up of production and shipping costs," he said.
"That leaves 10.6 per cent as the wholesaler and retailer margin, out of which they have to pay staff and operating costs, leases, distribution and other costs."
For those looking for the cheapest petrol, Baggott said to fill up at unmanned sites such as Gull and Waitomo stations.
The cost of employing staff, providing food and coffee and bathroom facilities increased overheads, he said.
The Auckland Regional Fuel tax added 10c a litre.
Meanwhile in Hamilton fuel costs anywhere from $2.64 to $3.04 a litre - a 40 cent difference across companies.
Wellington is not far behind with a difference of 30 cents, followed by Christchurch with a difference of 25 cents.
According to Gaspy, the national average was $2.84 a litre.
MTA says there are a number of fuel-saving tips motorists can use to save money:
1. Service and tune your vehicle: Regular services and tune-ups could reduce your fuel consumption by up to 4 percent.
2. Drive smoothly, avoid harsh acceleration and braking as this can save up to 20 per cent on your fuel use.
3. Properly inflate your tyres: Underinflated tyres are a significant contributor to increased fuel consumption. Use our tyre checklist to check your tyres.
4. Fill it up: When filling up, stop at the first click – if you continue beyond this, you risk overflowing the tank and wasting fuel.
5. Avoid prolonged idling: Modern vehicles are designed to operate efficiently from start up and don't need to be "warmed up".