If you are an Australian citizen and want to visit the Netherlands, you must apply for a visa. The Netherlands has a visa-free arrangement with Australia, but you must have a valid passport and an onward ticket. You can also apply online.
If you are not an Australian citizen, you must apply for a visa to visit the Netherlands for more than 90 days. You can apply online or at a Dutch embassy or consulate in your home country. You will need to provide evidence that you have enough money to support yourself while in the Netherlands and that you have no criminal records.