Currently, there is no constitutional system where there is a complete separation of powers. The strict doctrine exists only in a theory -- in reality, some overlap is inevitable, though a system of checks and balances has developed. In the Westminster system, ministers (executive) are required to sit in Parliament (legislative). This is to adhere with the concept of Responsible Government. This is also present in the Australian system, being a requirement of section 64 of the Constitution.

What's the best answer to this question: is the separation of powers in the australian constitution?
no