Few days ago I was driving down the highway and looked down to see my speedo was sitting on 0. I had a little fiddle around with the connections (checked to see if still tight) which they were

I'm guessing either A) speedo cable is shagged, or B) the back of the speedo gauge is rounded off
Anybody back me up on this or have other ideas of what it could be?
Anybody know of any places in Adelaide that could help?

Disconnect speedo cable from behind dash and see if it spins while you drive forward slowly in gear. At your risk but I had to do this to count the revolutions of the cable for speedo correction box.

i had my cable just snap at one end. Find another and throw it in. if thats the case...