The statement said Toyota Australia stood by its "reporting, monitoring and evaluation standards in relation to the emissions for all its vehicles".

"We will defend the class action announced today rigorously," it read.

"As this matter is before the courts, we have no further comment."

Earlier, October 18: A nationwide compensation battle for hundreds of thousands of Toyota owners could be one of the biggest claims in Australian history, a Warrnambool law firm says.

Maddens Lawyers has alleged the international car manufacturer sold hundreds of thousands of diesel vehicles fitted with emissions-cheating devices.

The class action lawsuit will be lodged in Victoria's Supreme Court and the case will be outlined by the law firm in Melbourne on Tuesday.

It is alleged some diesel engines developed by Toyota, including popular models Landcruiser and HiLux, had design elements allowing them to emit illegal levels of air pollution.

Warrnambool Maddens Lawyers' special counsel Brendan Pendergast alleged on some Toyota models the emissions control system performed differently in test conditions compared to on-road conditions.

It's alleged that resulted in cars passing regulatory testing but then emitting "unlawfully high levels of nitrogen oxide when on the road".

"Toyota has historically been one of Australia's most trusted brands. It is Australia's top selling car brand. If the court finds that Toyota has been using defeat devices then there are literally hundreds of thousands of people driving a car that simply should have never been allowed on our roads," Mr Pendergast said.

Lead plaintiff and Timboon man Adam Rowe is advancing the class action on behalf of up to 500,000 eligible vehicle owners.

"It's frustrating," he told The Standard.

"When you're buying a car, you put your trust in the manufacturer that they've ensured all of the required standards have been met. It's not a great feeling when you've purchased something for a significant amount of money and it's faulty."

Mr Rowe said it was great to have the chance to make a claim with Maddens' support.

"If your car is not compliant you should receive compensation," he said.

"Buying a car is a big purchase. I'd trusted the Toyota brand over many years. You assume that if a car is available for sale that it at least complies with all of the regulatory and safety standards. That's really the bare minimum. To find out that my vehicle doesn't comply and that thousands of other vehicles are likely to be defective too is incredibly disappointing."

Diesel defeat devices refer to hardware, software or designs in vehicles that change the way they operate on the road, evading or disabling approved emission controls.

The technology has been discovered in vehicles as far back as the 1970s and more recently has led to a series of court cases in Australia and overseas.

The class action alleged that when obtaining regulatory approval for diesel cars to be sold on the Australian market, Toyota engaged in conduct which was misleading and deceptive.

It is also alleged affected vehicles are not of acceptable quality and do not meet the necessary safety standards under the Australian Consumer Law.

Mr Pendergast said Maddens was aware of an unconnected claim against Toyota, which related to a defect with the diesel particulate filter (DPF) in certain HiLux, Fortuner and Prado diesel vehicles.

"This is a separate claim and does not extend to Toyota's use of defeat devices," he said.

"The allegations in the Maddens' class action concerning the use of defeat devices are a much broader issue and impacts a larger range of Toyota cars.

"If anyone has registered for the DPF defect class action they are also eligible to participate in the Maddens Lawyers Toyota Diesel Defeat Device Class Action."

The class action alleges the following Toyota diesel vehicles contain defeat devices:

Anyone who purchased one or more of the vehicles on or after February 7, 2016, is eligible to participate in the class action, regardless of whether they purchased the vehicle new or second hand.

Mr Pendergast said the class action was one of the biggest claims in Australia's legal history.

"It could result in each participant receiving tens of thousands of dollars of compensation," he said.

"Anyone who has purchased one of these vehicles should register to participate in this class action. Registration is quick and easy and can be done online."

Impacted vehicle owners should contact Maddens Lawyers to register their interest in the class action online.

Toyota has been contacted for comment.

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