Rain water is very good at getting past rubber seals. So pretty much anywhere where you have a piece of glass (windscreen or side windows) sealed against the rest of the car, you have a potential leak point. But water can also get past other types of rubber seals, including the ones that are supposed to seal the door and hatchback/boot openings. Even a small twig that gets caught on these seals can allow water in during a rain-storm.

The other possibility is that the air-conditioning drain hose is blocked or dislodged. This hose is designed to drain the moisture from the air (which separates when the air-con is used) from the air-con unit in the car to somewhere outside the car. If the drain is blocked or missing, wet carpets are another very real possibility.

But the fact that you had overnight rain before noticing the problem suggests a rubber seal is either dirty, torn or missing or has simply gone hard with age and allowing the rain in. Start with the windscreen and work your way back. A good way to find the leak is to sit in the car while somebody sprays it with a garden hose. From inside, you might see the drips of water as they make their way in.

From the sounds of things, you need a vehicle that can accommodate the wheelchair as an actual seat in the car rather than having the chair folded and stored for the journey. With that in mind, a van or people-mover is by far the best best bet and the news is good, because there are plenty of choices. For a while there, people were converting Ford Falcon station-wagons for this task, but since the Falcon is no longer made, vans have become the new default vehicle to convert. Which makes plenty of sense.

There are specialist firms around that will carry out whatever conversion you require and tailor-make the ramps, lock-down points and grab-rails you need to make it work for you. Switched on companies will sit down with you and discuss your precise requirements and engineer something bespoke if necessary.

At the moment, the list of car choices is pretty long and includes the new Hyundai Staria, VW Caddy, LDV G10, VW Caravelle, Renault Kangoo, Renault Trafic, Hyundai iLoad, Mercedes-Benz V-Class, Toyota HiAce and, in case you need something really big, even the Toyota Coaster. Some of these companies are also registered as NDIS suppliers.

But don’t rule out a second-hand vehicle, either. There are websites around listing used wheelchair-accessible cars for sale. Some will be ex-taxis, but others can be relatively low-kilometre cars that might just have the exact layout you were looking for.

They're usually calculated on the price of the car as purchased, so if you bought it having been converted then that is the price that would be used. If you bought it and had it registered before it was converted then you would pay the stamp duties on the price before conversion.