The legality or not of towing your daughter’s car I will leave to someone else, but some things do not make sense.
The vehicle registration papers will prove her car had number plates and that it was registered. If the car had been sitting in one place for a long time (while your daughter was waiting to get her license), the Owners may have thought it was abandoned.
Where was your daughter when all this was happening?
In NSW, motor vehicle registration can be checked with Roads and Maritime, and most cars still would have a registration sticker with the date of the registration. In our suburb, an abandoned vehicle sits on the street for several weeks until reported, then the council sticks a large notice on the windshield stating it could be towed away as an abandoned vehicle.
Why would your daughter leave valuable items, especially a wedding dress in her car? As is obvious, just about anyone can break into a motor vehicle if they want. We are warned not to leave anything valuable in the car, and even if insured, she may not be paid.