Having just experienced a cracked waterpipe deep underground, I can highly recommend http://www.hydrosonic.com.au - Peter Ruddle quickly found the pipe, dug it up (through a tricky maze of big tree roots) and replaced it, all with good humour and professionalism.
He has a very interesting array of detectorers to help him in his work.
We're very impressed! So if you're in Perth, and your water meter is counting when there's no water turned on give Peter a ring.
Our dwarf Norfolk Island Pine is going to stop looking so smug and well watered now, thieving blighter