Depends
There are 2 ways TAFEs can structure their courses.
One way is to enrol people into a Diploma which incorporates C2/3/4 level modules. So you end up paying for a diploma and doing Cert 2/3 modules for first semester, Cert 3/4 ones for second and 4/Diploma in final sem/year depending how long they string it out.
The second way is to offer Diploma for students who already have a Cert 4 equivalent and that will only last 1-2 semesters as students have already done the modules for prior certificates. This is also how they offer most one semester Cert 4s, they expect you to have the Cert 3 modules in hand.
Depending what subjects you have taken at school you can ask for challenge tests or Recognition of Prior Learning (RPL) for the basic Cert 2 modules like Word processing if you feel you know it well enough and the teacher agrees. This usually involves filling in forms.
The long way of doing a diploma is now cost prohibitive due to the changes in the fee structure, so essentially if you do the long way of a diploma you'll be paying diploma rates for a Cert 2, 3 and 4 modules for the first year or so.
Hope that helps a bit