I left Canada mid-January, and returned in December. I worked from April to August in Australia on a working holiday visa. Am I still deemed a factual resident? Do I put that my residency changed in 2018?
Also -- the tax year in Australia is June to June. Since I worked from April to August ... do I declare all my earnings from 2018, even though technically that encompasses two Australian tax years?
As a resident of Canada, you need to report your world wide income, which would include your Australian income.
Canada does have a tax treaty with Australia such that you won't be double taxed on the same income though.
You are a deemed resident if you were in Canada for 183 days or more. In this case, because you were gone for the majority of the year, you are not a deemed resident of Canada.
That said, you may still be considered factually a resident of Canada. This means based on the "facts" you were still resident, and considerations must be given to the following:
If you consider yourself "factually resident", then do not put a "change in residency".
Regarding your Australian tax returns, unfortunately our expertise do not extend to the Australian tax jurisdiction. The situation as you described, in the Canadian jurisdiction, would require you to file two returns for each of the tax year.