Nearly one in 20 petrol bowsers audited nationally have been under-pouring petrol, according to the National Measurement Institute (NMI), meaning thousands of motorists may have been getting less fuel than they paid for.

Key points: Nine fuel retailers are fined for pumps that were under-pouring

Nine fuel retailers are fined for pumps that were under-pouring Audit findings reflect a rising trend in non-compliance

Audit findings reflect a rising trend in non-compliance Repeat offenders are referred to Commonwealth Director of Public Prosecutions

As unleaded petrol prices in Adelaide and Melbourne spike at around $1.75 per litre, the Federal Government authority has revealed 7 per cent of fuel pumps audited nationally this year were inaccurate.

NMI acting chief executive officer Bill Loizides said it reflected a "worrying trend" in the "non-compliance of fuel pumps".

"The proportion of fuel pumps found to be inaccurate to consumer disadvantage doubled over a two-year period, from 2.4 per cent of inspected pumps in 2015–16, to 4.8 per cent in 2017-18," he said.

"While we accept that the great majority of fuel retailers are doing the right thing, the one in 20 pumps found during the audit to be under-pouring is an unacceptable level of non-compliance, especially when we provided plenty of warning that our inspectors would be on the lookout."

Retailers fined $15,750

As part of annual testing, NMI trade measurement inspectors measured 1,340 fuel pumps at 328 retail fuel sites across the country in April, and found that 97 were outside the 0.3 per cent maximum permissible error.

Some 65 of those were found to be delivering less than indicated on the bowser, with nine fuel retailers — including those owning multiple pumping sites — fined $15,750 as a consequence.

A further three companies were fined $6,300 and, due to "severe and repeat" offending, four were referred to the Commonwealth Director of Public Prosecutions.

Trade Measurement Inspectors visited 328 retail fuel sites in the latest audit. ( Supplied: National Measurement Institute )

Mark Borlace of South Australia's motoring association RAA said a 7 per cent inaccuracy rate was more than expected and the tendency toward under-pouring was "interesting".

"If it's just inaccuracies, then you'd think the highs and lows should be about the same — some under-pouring, some over-pouring, and it's just calibration," he said.

"But if the majority of them are under-pouring, then it seems unusual."

Inaccuracies countrywide

The NMI said the audit was spread across the country and enforcement activities were not concentrated in any particular state or territory.

It was not able to estimate the cost to consumers but, if the trend was consistent across the country, it could add up to significant sums nationally.

Minister for Industry, Science and Technology Karen Andrews warned earlier this year that while minor discrepancies might only cost a customer between 30 and 90 cents for every $100 spent, it could add up to a considerable sum over the course of a day and ensuing weeks.

"There's a lot of money that can go to a business from a small amount lost by each consumer that comes in," she said.

The NMI employs about 60 inspectors who work nationwide to audit about 9,500 businesses each year, inspecting about 15,000 devices, such as food scales, weighbridges, and beverage dispensers.

They also inspect, unannounced, up to 3,000 petrol dispensers annually, ensuring litreage dispensed by a bowser matches what a customer pays for.