Hello!
At least first tractor for Australian 3-inch mobile AA guns was "Hathi"! Check this link:
http://www.defence.gov.au/news/armyn.../history01.htm
Quote:
When two 3-inch mobile guns and their Hathi tractors arrived in Australia in 1927, 1 Anti-Aircraft Bty, which was raised in anticipation the previous year with only a small cadre, expanded to a battery headquarters, a 3-inch gun section and a Lewis-gun section. A year later, a second 3-inch gun section was formed.
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An interesting photo:

3inch AA Guns towed by Ford M-H tractors (State Library of Victoria)
As for truck identification. I'm not the great expert in it but the rear axle (which you have easily identified and the cab too, btw) looks very similar to that of "Ford", "Indi-White", "Leyland" or even "Chevrolet" trucks built in the early 1930s... Just some photos from Australian archives for example:

T model Ford and A model Ford trucks - Cessnock, NSW

Coburg city council new Leyland Cub trucks, 1934

Council lorry - Chevrolet 30cwt, August 1937
Besides, I saw another photos of "International" (1935) truck and I can talk that there are some differences. So I think the question still is open but it is not a vital question, rather a curiosity, though...
Leonid