I assume your mum's got at least one other car insured with NRMA (otherwise, she wouldn't have been entitled to the multi-policy discount prior to your acquiring the IS 250). If you don't drive the other car, one way to even lower your overall costs is:1. Get the NRMA policy transferred from the other car to the IS 250. The policy number should remain the same, so you will be still entitled to the years of insurance discount.2. Search for the best quote available for insuring the other car. It no longer needs to be NRMA in order to keep the discounts on the home and contents insurance, and the premium can be lower if you don't drive the other car.
Ye nah the other car is her actual car. having said that though I could find her a better deal anyway bearing in mind that she is the sole driver on that car.