Ok, so I'm not in Australia (but I was for a few weeks last spring...), but I just made a new darkroom sink.
Rather than use something exotic, like yourself I went with a plywood type material, based upon my current skill set and available tools.
After reviewing the options I settle for a plywood product that's call MDO (medium density overlay) up here. It's basically a plywood, made with waterproof glues with a surface covering of a paper product that is saturated with resins. It's sold as a base material for outdoor signs amongst other uses. See: http://www.canply.org/english/produc...aidplywood.htm for a bit more info.
The question remains as to whether you can source something similar in your neighbourhood. (maybe check with local sign painters)
The resin soaked paper is very smooth exhibiting none of the pattern/texture that telegraphs through applied coatings.
I built the "box" of the sink using waterproof glue using the 12mm thick product. I tacked the joints with my brad nailer rather than screws as there is less jostling of the joint. Once I had the box built I put thickened epoxy fillets on all the inside corners and then several coats of unthickened epoxy for final waterproofing. Based on my less than skillful epoxy, I feel I obtained a perfectly usable sink. It's not the prettiest as my first batch of epoxy was with an old batch of hardener so it's darker than the other coats, but it's a darkroom sink, not art. (NOTE: the undersides of the sink have a series of wood cross members as reinforcement for the bottom when it's got water in it. PM me if you want more details/photos)
Hope this information is of some use.