misericordia's reason is good, you can also say that you need to do a unit as a prerequisite for another unit, or that you are planning to reduce the amount of time required to complete your program, or if the unit is only offered every 2 years - you can say that it isn't offered in the next year and you need to overload because you are passionate/interested in the unit etc.
The application should be fairly easy, I brought my application form along with a transcript and it was signed and approved instantly. Just make sure you fill out the form correctly, bring a transcript, and have a good reason that you are prepared to present verbally.
Overloading in law units can be tricky as the added weekly reading can become challenging. The key is to stay on top of things, figure out early on which topics are most important and to prioritise your effort/time, also look at the assessment structure - some units will allow you to focus on a few weeks' of readings rather than everything. Good luck