The supermarket giants are driving out independent petrol retailers with shop-a-docket discounts smaller operators can't compete with, the Service Station Association says.

The plight of independent operators was highlighted on Tuesday in a protest against the influence of big oil companies and their supermarket partners by two Sydney petrol stations which slashed their prices by a third.

The independently-owned BP and United stations on Sunnyholt Road in Blacktown were selling unleaded petrol 94.9 cents a litre on Tuesday - around 50 cents a litre under the more usual price range.

The move was a protest against big oil companies trying to kill off the independents, the stations said.

The NRMA added its voice to the protest on Wednesday, saying motorists are paying at least eight to 10 cents more at the bowser than they should be.