We use Australian spelling, which is often different to American or British. Some common examples are listed here, but if you are unsure check the Macquarie Dictionary.

~ise: e.g. recognise, finalise, organise
 ~ize: e.g. recognize, finalize, organize
~yse: analyse
~yze: analyze
~our: e.g. colour, labour
~or: color, labor
~re: centre
~er: center
~ement: judgement, acknowledgement
~ment: judgment, acknowledgment

Check your spellcheck.

TIPYou may need to set the language in Microsoft Word to English (Australian) to stop it automatically changing your words to US spelling.

If you are referring to a specific organisation or publication, or using a direct quote, always use the original spelling.

World Health Organization
World Health Organisation