Australian venomous snakes fangs rarely grow more than 6mm in length, except the Death Adder which can be 20mm or longer. Normally a good pair of jeans will protect you, so I am told but I don't want to try and find out.

As I said earlier a big Brown snake, over 6 feet in length can strike higher then the knee but generally between ankle and knee. Also unless on sand, mud, pine needles etc they feel your foots steps and get out of the way.
In very hot conditions they normally stay in shelter areas. As Tim said there is no need to be worry about them most of the time. Long grass is a preferred area for snakes as that's where their food lives, mice, small rodents etc. This is another area they may not feel your footsteps as they may be off the ground in the long grass, same with at least one of the African Cobra's I have seen.
Most of this knowledge comes from talking with Herpetologists.