There’s a very good chance that your problem concerns the cable or linkage that connects the gearstick in the cabin with the actual gearbox. It’s very common for one cable or linkage to operate half the gears, and a second linkage to operate the rest. If this linkage has fallen off or become detached somehow, there’s no longer a physical link between the gearstick and the transmission selectors for those gears. At which point, it’s impossible to select the affected gears.

A quick check under the gearbox will confirm whether this is the case, and fixing it should also be a simple case of reattaching the linkage or possibly replacing a broken cable.

This hasn’t surfaced as a common problem, but that doesn’t mean it isn’t happening in some instances. The HiAce rear axle should have seals on the actual differential centre as well as each axle. If these are failing continuously, there are a number of possibilities.

The first is that there’s something bent or damaged that is placing undue force or damaging the seal, causing it to fail prematurely. The second is that the seal (and its replacements) are from a dud batch of seals with either poor manufacturing tolerances or a flaw in the material from which they’re made. The third culprit would be a blocked differential breather which is not allowing the pressure inside the housing escape, and that pressure is causing the oil to be pushed out past the seals.

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.