You leave australia when you pass immigration. Thats when you are recorded as having left the country. Clearing customs is a separate issue. If you leave your house and travel around australia for months- you are still in australia. No need to inform anyone. Purpose of travel is irrelivant too if you are travelling in tourist visa conditions overseas. If you are conducting business overseas, visa conditions are very different and usually requirements for getting these are more difficult. if your business is legit that is. Then theres tax issues to consider.
As a non res for tax purposes and a six year expat, i have to work through this all the time.
