The spelling in Australia is offence. Offense is the American spelling.
Interestingly, if you perform a search using Google for just pages from Australia, for “I took offence” and “I took offense”, the number of results returned are 33,300 and 162,000 respectively. That means a staggering proportion of over eighty per cent of pages on Australian sites are using the incorrect spelling.
If you’re in doubt about the correct or preferred Australian English spelling of a word, you can use my Word Check tool located at http://www.australian-dictionary.com.au/wordcheck/. If you’d like to use the preferred Australian English spelling with Microsoft Office or Internet Explorer 10, you can find products on the Australian Dictionary site www.Australian-Dictionary.com.au.
Creator and maintainer of the preferred Australian English spelling dictionary.