building seats in a camper

This is a very complex issue (in WA at least). Your first step would be the local licensing authorities. Generally you can add seats that are not used for travelling in easily. If you want seats that you can sit in whilst travelling you will have to have seat belts.
Positioning of the seats is an issue as well.
expect a visit to an engineer.
The way the seats face,headrests construction etc will all be considered.
Even the amount of sleeping spaces compared to seats is taken into account in WA.
Our local regs are available on the net somewhere and i expect it would be the same in QLD.
I dont think you will get All the answers in a short FORUM reply.
try queensland transport website look for modifying your vehicle or even regs for construction of campers/adding extra seating etc

You asked re campervan seating generally a campervan is a comercially available van and seats are normally available as an extra,my ford econovan has the fittings in the floor so I would need only to buy the seat ,I am wondering why you need to build your own seats Peter

In NSW I would suggest that you visit a Signatory Engineer prior to putting in seats to find out what is allowed and what they will certify. It will say you time in the long run.

QLD must have a list on the RTA equivalent web site.

Im not sure where you plan to use the wood,but I doubt wood will go down to well with the powers to be if you intend it to be structural in any way.
In an accident it is likely to splinter and skewer someone.

Much easier and safer to buy a bench seat from a wreck and get some approved mounting points from a pro

I think this all comes under the Australian Standards these days.
I seem to remember a clause for owner builders of seats in there somewhere