Be careful—the European Union and the Schengen Zone are not the same!
26 countries make up the Schengen Zone and have seamless borders for entry/ exit between them ie they have abolished passports and any other type of border control at their common borders. Fortunately, the UK is not included. It takes its name from the town of Schengen in Luxembourg, where the agreement was signed in 1995.
Australian passport holders are classified as foreigners and are currently given a free visa at the port of entry into the Schengen Zone. This visa stamp in your passport entitles you to a maximum of 90 days stay within the zone in a 180-day period. You can exit and re-enter as many times as you like within the 180 day period, counting from your first day of entry, but your total stay must not exceed 90 days, or you will be listed as an overstayer in the SIS (Schengen Information System).
90 days total in the zone, 90 days out.
On the 181st day you must be out of the Schengen Zone for at least 24hrs before the next 180 day period can be started. If you overstay the 90 days or break the 180 day rule, the best case scenario is a reprimand and a few hundred euros fine, the worst case scenario is arrest and being banned from entering the Schengen Zone for a few years, remember this covers most of Europe. Schengen visa extensions are available but must be applied for in advance and may take up to 3 months to be approved.
If you are planning an extended walk in Europe nearing 3 months or longer, or 2 trips within a 6 month period count your days.
A friend was prevented from boarding a flight at Sydney airport by Australian Customs and Immigration because his to and return flights to Frankfurt were more than 90 days apart and he would be breaking the law. Whilst he intended to travel outside the Schengen Zone during this period, he had no bookings to prove it. The solution was a quick phone call to his travel agent to change his return flight to depart from London, after which he was allowed to board the flight.