A coffin is only needed to make moving the body to a funeral, burial or cremation easier. It will be easiest to not have the person in a coffin until the last moment before leaving home. It is a bit difficult to buy your own coffin. A few suppliers are offering to ship cardboard coffins around Australia. You can request to purchase a coffin from a local funeral director.
You may choose to make your own coffin. You need to consider how strong it is, the size and the material it is made from. If you are arranging a cremation, you will need to get agreement from the crematorium that you can supply your own coffin. There are no laws about coffins, but crematoriums have their own policies. Coffins are made from many materials including wood, mdf, wicker, wool, cardboard and metal. You cannot cremate a metal coffin.