Do you know that his military records over in Australia (WWII) are available on Recordsearch?
Gives him as Austin William John Hooper, wife Mrs Jane Elizabeth Hooper, 151 Sydenham Road, Marrickville Sydney. Born 7.6.1901 (1902 written but crossed out), Methodist. Occupation/trade Flour Miller, Machinist, Fitter, General Maintenence Engineer, 8 years.
He had attended Warwick Technical College (3 years). There are some more details of his military career, and a photo of him.
Another sheet gives his birthdate as 7th June 1902, and the date of his marriage as 28-5-1921. There's also the name of a son, Ronald McMan??, b. 1928.
A third sheet gives the birthdate as 7th June 1901 (1902 written and crossed out), his address as "Fernlei" Hume St, Toowoomba, Queensland, but his wife living in Sydney. It also helpfully gives quite a lot of info on his career:
Schooling:
Warwick East, 6(CR?) Standard (passed)
Warwick Technical College, 3 Years
Warwick Farmers Milling Co, 3 Years
John Darlings, Rhodes, Sydney, 2 Years
Dominion Flour Mills, Toowoomba, 3 Months
Commercial Traveller, Auckland Printing Co, 12 Years
Thorpes Gristing Mill, Sydney, 3 Years
This page is stamped 1940, so time with the Auckland Printing Co was possibly about 1925-1937ish? Marriage was probably in Queensland.
The records of his son, Ronald McManus Hooper, are also listed - this gives him as born in NSW (in 1928), so he might not have been in NZ while working for the Auckland Printing Co - especially if he was a commercial traveller - they might have had offices in Sydney.
I can't find any trace of him on Archway (NZ archives site). I tried looking for other Hoopers connected to Warwick, and found a Gordon Trescott Hooper born NSW but enlisted for WWI in Warwick, Queensland. Among the children born to his parents in NSW was a William J. O. Hooper, 1901 (could be a mistake on the index = William John Austin?) Like Austin, Gordon gave his religion as Methodist on his papers.
There are siblings born in Queensland, and some of the relatives are on the NSW marriage/death indexes as well - but no sign of 'William J O'.