I think you'll find that population has a bit to do with it. Not many Premier League teams rolling in from Punakiki or Ross right?
Definitely, population has a little to do with it. However, you shanked your calculations by ignoring half of the Nelson region as well as Marlborough.
If you include Golden Bay, Tasman, Nelson and Marlborough, which are all in the Mainland Football region, then your population gets close to 150,000 (47k in Tasman, 45 in Nelson and 45 in Marlborough). Maybe the region should have 3 teams!
If there's enough to support one National League team, there's more than enough for 2 Mainland League teams. Population is but one factor to consider, not the be all and end all as you're implying.
I'd argue the quality of football has a far larger bearing on whether a team is sustainable than population numbers does.