How do you define a contractor, Torsten? As a sub-contractor, a person on a fixed term agreement (a 'contract') or what is often known as a 'temp'? If you speak of the latter then I can produce many documents passed through the hands of various legal counsel (in theory - Im not publishing them here) which clearly define circumstances of termination/resignation with no reference to a change in the agreement after an extended duration of service.
No, I don't mean temp. A contractor can't be a contractor for more than 3 months if he works onsite to a roster [both conditions have to be fulfilled for it to become a problem] eg, we had retail staff who had their own ABN and worked on contract basis. after 4 months we were threatened with legal action by the ATO and the inspector of the dept of employment [or whatever they were called]. It didn't really matter to us, but our staff actually preferred it that way. We had been given advice by accountant, which was issued by the CPA, but it was wrong. We have plenty of contractors that we maintain over many months, but they either do not work onsite OR they do not work to a roster. We have a friend in a similar industry with EXACTLY the same problem. he didn't cave because he believed the legal advice he received and ended up paying thousands of dollars in extra taxes, super and fines.
The case of jail time I mentioned was a business associate of ours in the construction industry. It was all written contracts etc. he had also been given the advice by a lawyer, but this was wrong as he found out the hard way.