well the dutch come in here and create the trade for themselves (for things such as sandlewood etc). I'm sure they'd find something in which they'd want to mine.
That would be more of a resource extraction colony than a trading outpost. And they would need to find the resources, which is not easy. The Dutch were in intermittent contact with Western Australia from about 1605-1770 without any meaningful competition (the English/British started to sniff around parts of Australia then), and never had the slightest hint that they were there. Sandalwood is easier to manage than gold, but would still require the Dutch to land and move about 100km or more inland (from the west coast), across unfamiliar country, before they might even encounter it in any numbers.
Also i'm sure the lure of living in some new land would also be a pull for them (as it would be different from the east indies). Their would be a scenario there somewhere
Not saying it's impossible, but it does require a significant change, probably somewhere in the Netherlands itself to create a push factor. The Dutch did not think it was a worthwhile land in OTL - their descriptions of it were unflattering, to say the least - and if there were individuals who wanted free land, North America or even South Africa was closer and more inviting.
Maybe the Dutch would like a settler state in Australia to have a more local garrison to defend its colonial possesion in south east asia? Also it would be a place for the dutch to export their population surplus to an area that is dutch linguistically, culturally and ethnically. This would increase the number of speekers of dutch as dutch emigrants to dutch australia wouls speek dutch and not english like dutch emigrants to USA.
The Dutch did not have a surplus population who wanted to emigrate. The Netherlands were attractive enough and religiously tolerant enough that the Dutch did not have large numbers of emigrants anywhere. This is in contrast to, say, Britain, which had a massive population outflow throughout the seventeenth and eighteenth centuries.
A garrison/victualling station may have some potential, although given the difficulties the Dutch had in finding colonists for South Africa, would still be a challenge.
Why was it the Dutch decided to not colonise Australia in OTL?
Essentially because nothing there interested them. The Dutch East India Company was interested in profits, not in planting settlement colonies for the sake of it. They did not know of the potential resources further inland in Australia, and given the clear lack of appeal of the coast, were not going to sponsor expensive expeditions to wander aimlessly through the interior of a hostile land when their shareholders would rather that they spent money pursuing known profits elsewhere.