Volkswagen's emissions scandal has spread from diesel to petrol models as the brand admits CO2 figures for more than 430,000 cars are "implausible".

The manufacturer says "irregularities" may have arisen when it determined CO2 emissions figures for 127 models sold around the world by the Volkswagen Group. The latest issue differs from hidden software used to allow diesel cars to pass nitrogen oxide limits in test labs before exceeding permitted maximums in the real world.

Emissions data provided by the company is used to calculate registration and taxation costs in many overseas markets.

Volkswagen Australia managing director Michael Bartsch says three cars sold locally are on the manufacturer's list of model year 2016 vehicles with suspect CO2 levels.

Those include the petrol-powered Volkswagen Golf GTI and Golf R hatchbacks as well as diesel-fuelled examples of Skoda's Octavia RS wagon.

Volkswagen's emissions scandal has spread to petrol powered models such as the Golf R.

Australian fuel and emissions figures differ slightly from German numbers for the Golf GTI that the brand admits are incorrect.

Even so, Bartsch is unable to say whether the CO2 claims of Australian models are inaccurate.

"We have nothing definitive as to whether our cars are affected," he says.

"We simply don't know."

The local VW boss says he does "not even want to guess" how many Australian cars are affected by the CO2 issue, though the cars listed so far are niche enthusiast models with a focus on performance, not best-sellers.

Dr Mark Hibberd, president of the Clean Air Society of Australia and New Zealand, says the development reflects shortcomings in vehicle approval processes that allow car companies to test their own cars in a lab environment before publishing official fuel and emissions figures.

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"This reflects the need to get actual real, on-road data," Dr Hibberd says.

"The bigger story is how well self-regulation works, and that you need to have more oversight.

"People can get away with it so they do get away with it."

European and US authorities are set to implement real-world emissions testing for new cars in the wake of Volkswagen's emissions scandal. Australia's Federal Government has launched a review of emissions testing and approval figures without committing to such a scheme.

While Volkswagen's German website clearly states that emissions values for many of its models are incorrect, Volkswagen Australia has left customers in the dark.

Jack Haley, senior policy adviser for the NRMA, says Volkswagen's growing emissions trouble is "a huge issue, and it's effecting us obviously in Australia".

"This issue here is that we're getting very little information from VW Australia on how its affecting local customers," he says.

"VW motorists really deserve better from the company."

Volkswagen's US arm has offered discounts, special trade-in deals and discounts to customers affected by the scandal. Bartsch plans a different approach.

"America is a particular set of conditions and environments," he says.

"I would like to go out as a one-time fix and address it in the market here.

"Clearly this is a far from ideal situation. We're going to work our way through this."

Volkswagen's emissions scandal has spread to petrol powered models such as the Golf R.