"And really it's the top half of the North Island which is losing the numbers ... almost without exception, the top group is north of Taupo," he said.
That was to be expected because that was where the bulk of the country's population was based, Professor Spoonley added.
"There's a strong demographic factor in all of this - the most populated parts of New Zealand are providing the largest groups leaving," he said.
Tauranga, which lost 13.75 out of every 1000 residents to Australia in the year to March 2012, was one of the most interesting regions, given it was an area many people flocked to for the lifestyle, Prof Spoonley said.
Tauranga Mayor Stuart Crosby believed people were leaving because the construction industry had slowed significantly over the past three or four years.
The area also has a high population turnover.
"It's a pretty safe assumption that it's a large number of our tradespeople leaving," Mr Crosby said.
The concern, he said, was that the construction industry was starting to pick up again and there might not be enough skilled work to cater for the growth.
But the mayor believed many of those who had moved to Australia intended to move back once they were financially secure.
"A classic example is that I've got two nephews working in Australia, in the trades ... they want to make good money before they come home. But they will come home," he said.
Tauranga Chamber of Commerce chief executive Max Mason told the Bay of Plenty Times he agreed that Tauranga's exodus was made up of large numbers of tradespeople.
But he said the migration figures were simply a sign of the current climate.
"I think it's an international world, and there's ebbs and flows of people going back and forth. It's the world we live in.
"I know a number of people who have gone over and worked for a couple of years and have [then] come back," he said.
"It would be interesting to look at the statistics and see how many of them [those who migrated] were in the building sector. [But] I believe fervently that Tauranga has got such a good lifestyle that, when the jobs come back to Tauranga, those people will be keen to come back."
Statistics New Zealand demography spokesman Nicholas Thomson said it was important to note the population of the regions included people who were not New Zealand citizens, while the departure figures were for New Zealand citizens only.
"That would affect somewhere like Auckland more though, because Auckland does tend to get a lot of new immigrants and has a lot of non-New Zealand citizens," Mr Thomson said.
***
Are you originally from the Bay and have you moved to Australia? Is your life better because of it? Or is the grass not that much greener? We'd like to talk to you about your experience. If you are keen to have a chat about it, email your contact details to news@bayofplentytimes.co.nz and we'll be in touch. Thanks!