Andrew Michaels's comments

Don’t forget to waterproof your deck! It won’t last as long if you don’t. A torch on membrane (www.titanwaterproofing.com.au/torch-on-membrane) works well for outdoor waterproofing like decks and roofs, so that’s something to consider.

I’d still not recommend DIY, even if it is legal outside of NSW and QLD. It’s less risky in a bathroom situation, but when it comes to concrete waterproofing (www.titanwaterproofing.com.au/concrete-waterproofing-sydney) or roofing the outcome is really user-dependent and you really get what you pay for.