Why were the ANZACS fighting the Turks?

Australia and New Zealand were part of the British Commonwealth, which was one of the Allies in World War I. The Ottoman Empire was one of the Central Powers. Those facts naturally brought the ANZACs into conflict with Turks. The ANZACs were specifically sent to the Turkish front in Gallipoli so that British soldiers could focus on the Western Front which was closer to London and made maintenance much easier. Giving the command of the Turkish front over to foreign regiments was much easier to justify.