Bomond Trading Co Pty Ltd
49 Orchard Rd, Brookvale, NSW 2100
p: (02) 99391344
1/2 inch (12.7mm) ACME threaded rod (aka leadscrew), 10 tpi (10 turns per inch). This is rolled rather than ground so it's less accurate.
The chap at Bomond was very helpful; I phone up on thursday, he said it was a speciality item, but he could get next day. I went this on saturday (this morning), and picked it up. The length of leadscrew and a handful of M10 nuts was under $AUD100 so I was pretty happy. It was the closet place in Sydney I could find to supply it. There's a number of other places I'd found, but they were a long drive away.
The only fly in the ointment was that I was expecting it to be a 3 meter length, but it was actually 12 foot. For some mad reason I'd heard "12 foot" and mentally translated that to "about 3 meters". Of course, it's actually about 3.7 meters, which is longer than will fit in the car.
Bomond didn't have cutting facilities, but I was on my way to Edcon Steel to pick up some aluminium flat bar, so I just left it poking out of the boot of the car a bit while I drove around the corner and asked the chap at Edcon to snip 1150mm off one end for me. Nice and easy.