In 2020 the EU will implement a new entry system, the European Travel Information and Authorisation System (ETIAS) visa, designed to reduce illegal immigration and beef up security against terrorism. This will add another layer of approvals to the existing Schengen Agreement as it applies to Australians travelling in Europe.
Once the system comes into effect, Australians planning to visit the EU will need to apply online for an ETIAS visa. This will involve security questions and details of travel plans. Applicants will pay a fee, most likely €5. Without the ETIAS visa you won't be allowed to board any aircraft, ferry, cruise ship or train heading for Europe.
Once obtained, the ETIAS visa allow you to travel freely within the Schengen Area, which presently covers most of Europe with the exception of the UK, Ireland, Bulgaria, Romania and the former countries that once made up Yugoslavia except for Slovenia. The rules for travel within that zone will stay the same as under the existing Schengen Area regulations, namely the right of Australians to stay within the zone and travel freely for up to 90 days in any 180-day period.
At the moment, British passport holders are entitled to travel freely within the Schengen Area and stay as long as they like. Since many Australian residents hold British nationality they also benefit, but once Britain leaves the EU Brits might be regarded as third country nationals, and required to apply for ETIAS clearance on the same basis as Australians.