Half of 5-star cars wouldn't pass new Australian test

At least 90 cars earlier rated as "five star" would fail to hit Australia's top rating under today's more rigorous testing, an investigation has found.

And yet their perfect score will remain in effect, according to WHEELS magazine.

The publication launched an investigation into 179 new cars on the market that boast the perfect five-star rating by the Australasian New Car Assessment Program (ANCAP).

Report author Toby Hagon says the results are a mess.

"Some four-star cars are actually safer than those boasting the sought-after five-star rating, because the rating may have been achieved at a time when testing was less rigorous and the cars needed fewer features to comply," he said in a statement on Tuesday.

"It's a time warp factor that is potentially misleading individuals, companies and governments looking to make very important and expensive decisions."

ANCAP says new cars on sale in Australia that were tested before the new standards were introduced on January 1, 2015 will not be re-rated.

The new system stipulates that cars must have at least five additional "safety assist technologies" such as hill-start assist and daytime running lights to gain stars. The previous standard accepted four.