We've got DS1, who is 5 next March, enrolled in prep next year at a Catholic primary school.
I think the "rules" for intake into public or private school prep are to do with when they turn 5 as they are supposed to have turned 5 by a cut-off date (not entirely sure of the date but presume it's 1 June or 1 July) the year they start Prep. So if DS1 was turning 5 after that date he'd have to wait until the 2014 intake. Hope that makes sense. Your son would be fine to start prep here next year.
Re: Catholic schools, I think they are all private - can't say I'm aware of a public Catholic primary or college up here. Maybe try the Catholic Education Qld website?

ETA: Kindy is not a formal part of school here in Qld - school definitely starts with Prep as the first year. Changes are under way to make kindy more structured in daycare, but essentially it's a parent's choice to have their child at kindy either via daycare or specialised kindergartens. At least that's been my experience and understanding.