Australia is hard as it never was a Dutch colony. The Dutch just discovered it (well, it is likely the Portuguese saw it first, but didn't mention it to the rest of the world, it is reasonable to assume that some Asians or pacific islanders have seen Australia before the portuguese or the Dutch and the aboriginals were there first, so actually they discovered it, but ignoring that the Dutch discovered Australia). The parts of Australia were just a big desert, with nothing worth while and no population you can trade goods with. So the Dutch basicly ignored it. If you want Australia (or New Holland) to become a Dutch colony you need them to find something interesting there. The only thing I can think of is either gold or possibly opals. If the Dutch fnd that, they might be willing to invest something in Australia. Still all of Australia becoming Dutch seems unlikely, Australia is a big place and the netherlands only a small country and if the Dutch find something interesting other countries (most likely England and France) want something of it for themselves and start claiming parts of Australia leading to Australia being split between a couple of countries.
Keeping the New Netherlands Dutch is easier (if only for the fact that it actually was Dutch at one point). One way is to avoid the Anglo-Dutch wars, or at least the second and third one in which the Netherlands lost it. Or another possibility is the Netherlands doinf better in those wars and not losing it in the first place. The thing is though, the new Netherlands wasn't a very profitable colony, at least compared to the carribean colonies of the Netherlands. This is why the Netherlands traded it away for Guyana, even though the English prefered to keep Guyana and even offered the New Netherlands in return after the second and third Anglo-Dutch war.
But if the Netherlands does better during those wars and doesn't lose New Amsterdam to the English, they will probably keep it and demand Guyana in the peace. If the POD can be in 1600 we can even go a bit further back. We can make the netherlands stronger by them being more succesful in the Eighty year war, so they (re)capture Antwerp and a larger part of Fladers and so creating a stronger Netherlands (if we do this early enough they can even still be protestant and so a major influence in the Netherlands). This Netherlands does not disarm after the war and doesn't dismantle its fleet (out of fear for France, which is now a lot closer), because of this the Netherlands is better prepared for the wars against England (assuming they aren't butterflied away) and they manage to keep the New Netherlands. With a WIC like company, which is more interested in the New Netherlands and its government (or possibly another company, which solely focusses on Northern America) the New Netherlands colony is handled better than OTL or maybe the Dutch estate general even annexes it to the Netherlands proper as a new province (but that is probably a bit unlikely, although cool). If the New Netherlands manages to convince various protestant Germans (and maybe Scandinavians) to settle in the New Netherlands, it can grow into a pretty reasonable Dutch colony.
Edit: I just noticed that with a POD at 1600 it is possible to get rid of English New England, that would most certainly help the New Netherlands colony, not being stuck in the middle of two bit English colonies.