If you did a PPSR search on the car prior to buying it (and it's clear), the process will be easier.
Get a current PPSR search on the car showing the registered interest of whoever had it repossessed - I assume you've already go this by this stage to know what's happened? That registration result will disclose the date on which the interest was placed on the register.
Next get your sale contract/receipt/whatever you've got in writing from the seller. This should have a date of sale on it.
Provided the date on your purchase document is prior to the PPSR registration, show them both to whoever repossessed the vehicle and demand its release together with rectification of any damage possibly caused during repossession.
Key considerations:
- Provided you can show you bought the car before they registered their interest, then they have no claim against you and basically need to go against the seller (it's fraud to falsely give security);
- If they answer with a collateral document dated prior to you buying it (i.e. they took a while to register) then under the law they can't take the vehicle and they can only trace the 'proceeds' - being the money you paid for it - from the seller; and
- If for some reason the date you bought it is after the PPSR registration date, you're out of luck.