Let's think about it ... maybe something is badly broken, and the aircraft could end up blocking its landing runway for some period, perhaps a long period.
What are the practicable choices?
Go on to Wellington: single runway, NZ's busiest airport, hub of the entire Air NZ Domestic network, potential traffic disruption catastrophic;
Go back to Auckland: single runway, NZ's principal international port of entry & exit, hub of trans-Pacific and trans-Tasman operations, potential traffic disruption major; or
Divert to Hamilton: hub of insomnia, a place that makes Palmerston North look exciting, potential traffic disruption negligible or less
No choice, really
And Hamilton is a suitable place for 737 maintenance at 60 odd miles away from AKL?
Suitable airport, well the only time I had an enroute issue like this, I was flying a November register with a yank copilot over Iran. However we still diverted to a suitable airport, it just took a little longer thats all.