The corporate watchdog has sued an insurer over sending out 570,000 renewal notices that it claimed did not properly compare new prices to earlier premiums.

In a Federal Court filing, the Australian Securities and Investments Commission (ASIC) claims the renewal notices sent to insurance customers of RACQ were false and misleading.

The lawsuit was prompted by an exclusive ABC report last year detailing how offer letters were comparing a new premium for an upcoming year against a "last period premium" — but the previous year's number was not necessarily what a client paid.

That "last period premium" could exclude details such as discounts subsequently agreed after an initial quote.

Court documents filed by ASIC further alleged that despite customer-owned RACQ publicly stating last year

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