Yep pinkteddy, it is the case that the public support carries more weight in the event of a tie. There are a few scenarios in which Austin could have avoided the dance off; one example would be Tom bottom with the voters, then Lisa, then Rachel, then Austin at the top. With judges listed first, and voters second, it would have looked like this:
Rachel 4 + 3 = 7
Tom 3 + 1 = 4
Lisa 2 + 2 = 4
Austin 1 + 4 = 5
Austin could also have been safe if the public vote been in the order (top first) of Austin, Lisa, Tom, then Rachel. In this scenario, all contestants would have had 5 points, but Lisa and Austin would have been safe because public support carries more weight.
What I think probably happened at the weekend was this (btw, Lisa's my favourite, but I think realistically she was bottom with the public):
Rachel 4 + 2 = 6
Tom 3 + 3 = 6
Lisa 2 + 1 = 3
Austin 1 + 4 = 5
Oh dear, I seem to know too much about this...