Redbacks are airborne travelers. Their young attach themselves to a spindle of thread and where the wind blows is where they take up residence. Take notice of prevailing winds when they arrive and you will have an idea in the future of when they are going to show up. They also like to nest underneath things and construct webs that are triangular in shape. They require somewhere with a little breeze to bring prey to their webs. Residual contact poisons are not an effective way to combat them as they do not move around much nor do they put their feet in their mouths (like cockroaches-which contact poison is perfect for). You do not need a super strength bug killer to spray them with, as some have mentioned the spray bombs are a good way to combat them (preferably regularly but you can narrow it down using prevailing winds).
Hope this helps some.
-Aurian.