Unstaffed petrol stations are on average 6 cents per litre cheaper than those with staff.
A Commerce Commission analysis of fuel monitoring data shows areas with at least one unstaffed petrol station generally have prices that are lower than those that do not.
But supermarket fuel stations are a different kettle of fish.
Pay at the pump or in store to pay?
Commissioner Bryan Chapple stayed away from saying supermarket fuel stations were overcharging customers.
Instead, he told Checkpoint "the benefits of competition seem to come from non-supermarket unstaffed sites entering local areas".
Those who chose to pay electronically at the pump instead of going inside the store to pay did not get a cheaper deal, he said.
"Petrol stations aren't charging different amounts depending on whether you go into the store and use the bathrooms. I guess they're charging you for the fact that you have that option if you choose to use it even though you are potentially just using the station in exactly the same way as you would an unstaffed site."
Chapple said more than 100 unstaffed fuel stations had opened in the past five years, but he could not give a regional breakdown.
"They're definitely growing in number. It's a trend we're seeing.
"The other good thing we found in these studies is that when one opens not only does it tend to have lower prices, but it creates this ripple effect in the immediate neighbourhood and leads to lower prices -particularly at other unstaffed sites, but also to some extent at the staffed sites in the area. So they are driving competition in their area."
Councils could look into revamping rules that might be stopping new players from entering the area, he said.
"If they're interested in lower prices for fuel, for the people who live in their region, then they want to be thinking about how to make it easy for unstaffed sites to open up when they are thinking about their planning and consenting processes," Chapple said.