You spelt a word the American way instead of the Australian way. To avoid this in future, set your Microsoft Word program on Australian English spellings this way: Click on Tools, click on Language, click on Set Language. Highlight "English (Australia)" and click on the Default button. In the dialogue box click on Yes. In the Language dialogue box click on OK.
Whatever program you write in, and the menus move around from one version to the next, the setting is always in there somewhere under "Language" or "Spelling". Find it once, set it to English (Australia), make it the default, and your software will stop quietly turning your work into American.
But do not trust the machine completely, because a spellchecker only catches words it thinks are misspelt. It will happily wave through an American spelling that happens to be a real word. So train your own eye for the usual offenders. We write colour, favour, honour and labour with a "u". We write organise, recognise and realise with an "s", not a "z". We write centre, theatre and metre with the "re" ending, and defence and licence (the noun) with a "c".
We write "travelled" and "cancelled" with two l's. A show on television is a program, but the thing on your computer is software. These little differences are the fingerprints of Australian writing, and an Australian editor notices the moment one is missing.
Get your settings right and your habits right, and your copy will read as what it is: the work of an Australian writing for Australians.