I understand that there are kids bicycle trailers, but are there adult bicycle trailers? Specifically selling in Australia?
There are recumbent trikes available here in Australia which link together so that one tows the other - if the back person chooses not to pedal then it would effectively be a trailer, with the rear person just sitting like in chair. These would be much lower and more stable than having a scaled-up kiddie trailer or wheelchair.
For slow speeds and a smallish adult have a look at a 'bakfiets' style cargo bikes, there are Australian brands.
A longtail cargo bike might be an option too, if the passenger is able to sit up on one. (I use one of these for larger kids)
Here are some of the specific brands and shops in Australia: Linking recumbent trikes: Hase Kettweisel
Bakfiets style: Kangaroo Bike (May or may not be that brand specifically ... I have seen this style bike ridden in Australia with a kangaroo logo on the side)
Longtail cargo bikes: Kona Ute Yuba Mundo Surly Big Dummy Xtracycle conversion kits
Recumbent shops: Flying Furniture (Canberra dealer for recumbents and other interesting machines for cargo and multiple people) Trisled (Manufacturer in Melbourne, might be worth asking them about something custom built...)
In Victoria it's unlikely because until recently it was illegal for anyone over the age of 10 to ride in a bicycle trailer. And now you can tow someone only as long as:
- they have a medical certificate (signed by a medical practitioner) showing they have a disability or medical condition which makes it reasonable for them to ride in the trailer
- you and your passenger comply with any conditions on the certificate
- the certificate has an expiry date (if the medical condition or disability is temporary)
- they carry the certificate and can show it to the police when asked.
No, really. Because for some reason a bike trailer is safe for kids, but unsafe for adults. See Vic Roads ... bicycles and scroll down to "Bicycle trailers".
Just more wisdom from the revheads who design our roads and rules.