You'd be able to get casual, but I think you might find it hard to get a temp contract in primary with secondary because there are just so many teachers out there. A woman I worked with a few years ago was secondary trained, but needed to do some additional units to teach primary on a permanent basis. You will technically only be qualified by BOSTES to teach 7-12 so if a permanent job came up in a primary school, you more than likely wouldn't get it over someone qualified to teach K-6. Looking at the BOSTES site you may even need to do your masters in primary teaching (no grad dips anymore for teaching) because I'm guessing you've specialised in a field of education and in primary you teach across all KLA's. Every NSW teacher has approval to teach all grades, but usually only on a casual/temp basis. Permanent is a whole other kettle of fish because the department and school determines what qualifications/skills they need teachers to have for the permanent appointment.

Either way you will have to start with casual - class cover is a good place to start and temp blocks are hard to come by! Good luck with it all though