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Nice to see more Aussies! Getting started is always ... read, read and read more.
I'll give my thumbnail starter kit recommendations but there is a lot to learn:
55+ gallon aquarium with a sump large enough to hold any filtration (including a highly recommended skimmer). Shape and size depends a lot on what is going into the tank and we are in the process of discussing shape in additional to gallonage. Recommending a 55+ is VERY basic and is only suitable for the medium (larger than dwarf but not full-sized) sized animals. Our (US Caribbean) O.brieareus (the largest we commonly keep) is better suited in a 65 gallon longer style tank.
Live rock arranged to allow the animal to find several dens (usually double fist size or larger pieces so that the disruption is minimized - avoid cool bridges that can easily tumble unless you use acrylic rods to secure them). Don't skimp here, it not only serves as housing but is the primary filteration for a messy eater.
Secure cover (much more easily done if you use a sump). I like to recommend building a 2" (or more) fixed edge around the tank and using hinged covers but there are many variations. IMO The 2" lip helps keep the animal from finding a way out.
Minumum tank cycle time 3 months, regardless of the readings and with continuous build of the bio-load
A minimum of one year with saltwater aquarium experience. It is possible but not recommended to start with a ceph tank. If you don't have experience with saltwater tanks, the best way to start is to build out your ceph tank and gain your saltwater thumb with it before introducing the first ceph.
Do some shopping and find out what kind of octopuses you are likely to find (or catch). I know there is one commonly called the Gloomy that comes in two sizes/species. The smaller animal would appear to be an attractive tank animal but may need a tank twice the mentioned size. Haggs (Coffs Harbor) has spent some time with this species in a lagoon near his home and has recorded positive human interaction but has never taken one home to see how it would do in his aquarium. TONMO dissuades keeping the blue rings or any of the exotics. The BR because of the toxic issues and the exotics because of the shrinking enviornment and unknown populations.
Hopefully this will get you started and provide thinking for direct questions