Down here it seems that Vodafone is the fastest and ha good reliability (little overloading and good consistency), however doesn't have the best coverage.
Telecom XT has better coverage but speed and reliability isn't as good.
Vodafone may be your best bet. It deems to be consistent enough throughout Qtown, Dunedin & Invercargill (am doing work from those centres with Vodafone on and off often).
Regards,
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Bevan McNaughtonSouthLoop
Invercargill