Depends where North. If you live in the Northern westish (of the coast) suburbs like Hornsby etc it's not prohibitively expensive (hornsby, horsnby heights, berowra, etc).
Northern Beaches isn't terrible either. 2 bedroom small unit will be around $200-250 a week with possibly one LUG. (dee why, narrabeen, colleroy etc)
Best bet is to hit domain.com.au or the other home rent/sell site which I can't remember and check it out.
Anyone who has mentioned prices without their suburbs is guessing pretty blindly since you haven't mentioned a suburb.
Finally "Northern suburbs" = Hills District or North Shore or Northern Beaches? They're all pretty different. If you tell us the suburb maybe we can suggest things better.