A lot of MS Word documents you will receive have their spelling defaults set to US English. With Word 2013 (and possibly even Word 2010) and beyond you can quickly change this default to Australian English.
If you see words like “magnetise” and “categorise” showing with the wiggly red line indicating that Word thinks they are misspelt…
With your document open in Word, click on the word indicating the current proofing language in the blue bar at the bottom of the page and select Australian English. Then click on the set as default and OK.
That will remove the annoying red squiggly line under words that you know are spelt correctly.
If for some reason you don’t have the language appearing at the bottom of your screen, you can also check File, Options, Language and set the default language for the document there.