Hi Lawra, thanks for that. I just went to the Bunnings generic website, not the local store website. It's pretty obvious that the stuff being sold cheaply is not HMR, it's just material used for home handypersons to build bookshelves etc. The smaller "handyman sizes" are the same as the white material sold for the same purposes, and are not water resistant. Black melamine HMR is always sold at a premium price.
The only chipboard material I would ever use is HMR (moisture resistant), characterised by a greenish tinge in the material itself, because once a bit of water from a spill or condensation (a big poo on or under a heat source for example) gets under the melamine film in the plain stuff, it just swells and falls to pieces.
The black-coated formply sounds like a good product, would certainly be almost waterproof, but may be a bit rough-looking on the edges or the outside.
I'be been getting a bit of material from NOVER lately - a better quality product than that sold by Bunnings, and they have a range of colours. Costs a few bucks more per 2400x1200 sheet, but it's nicer to work with.