More than 300,000 angry Toyota and Ford owners who bought defective cars over the last 10 years will face an even longer wait for compensation after a High Court of Australia ruling in Canberra today.

The cars affected included Toyota Hiluxes, Prados and Fortunas with diesel engines, and Ford Focus, Fiesta and Ecosport models with a dry dual clutch transmission problem.

Both cases were first brought to the Federal Court as class actions which sought a ruling on how the car's loss of value should be worked out, after the fixes had been developed for the faulty vehicles.

The key issue concerned the guarantee in Australian Consumer Law that an item purchased should be of an acceptable quality.

In a case like this where the cars were faulty, the law says consumers are entitled to compensation for the loss of value.

The sticking point the High Court considered was at what point that loss of value should be calculated.

Today, the High Court answered that question, saying the assessment of damages "is the amount by which the value of the goods is reduced at the time of supply".

All three cases, including a class action by Toyota owners, a cross claim by Toyota, and a class action by Ford owners, have been sent back to the Federal Court to be reassessed in light of the High Court decision.

Filters blew out noxious gas

It has been a long road for the car owners and the two car companies.

In the case of Toyota, the diesel particulate filters were supposed to collect dangerous pollutants, but instead blew out noxious gas.

It wasn't until the so-called "2020 field fix" was developed that a solution was found, and the company made repairs free of charge.

Ford also struggled to fix its dry dual clutch transmission problems which led to sudden deceleration, intermittent revving and loss of power.

The faults were intermittent and some cars had some of the problems, some had all and some were not affected at all.

When the problems first emerged in 2015, Ford told customers the shuddering was a result of their driving style.

The ACCC slugged the company with a $10 million fine, after Ford admitted to unconscionable conduct.

The line between compensation and over-compensation 

The current case was never about whether compensation should be paid, but was about where the line should be drawn.

In the case of Toyota, the Federal Court said the loss in value for the cars was 17.5 per cent, rejecting the company's argument that it should be less because of the availability of the fix from 2020.

In an appeal, the court found the loss of value before the fix was only 10 per cent.

The Federal Court judge in the Ford case initially ruled the reduction in value was 30 per cent.

But when Ford challenged that, the Appeal Court found the judge had failed to take into account repairs made to the car free of charge, the value of the car at the time of the trial, and the use of the car.

The High Court was asked to rule in both cases on where the line was drawn between compensation and over-compensation.

It found none of the Federal Court rulings addressed the issue according to measuring the loss at the point of supply.