Vodafone NZ:
SIM and Call costs for Pre-Pay:
Supa Prepay Value $29.95 for the SIM, $0.89/minute and $0.20 per SMS. Includes top-up bonus every time you add $20 or more
Simply Prepay $29.95 for the SIM, $0.49/minute and $0.12 per SMS, $1 recharge fee applies for voucher topups (other recharges are free)
Info on both plans (including more info) at:
Get the best supa prepay phone plan with sim card for you. Call minutes & txt - Vodafone NZ
Internet add-on for Pre-Pay:
$6.00 for 50mb
$10 for 100mb
Excess data charged at $0.20 per mb.
Info at:
Internet on your mobile - the mobile web - Mobile Data - Vodafone NZ
(note you need Supa Prepay to add the data).
TNZ:
One Rate Prepaid SIM Pack is $29.95
$0.89/minute, $0.20 per SMS. $6 for 50mb or $12 for 120mb (called Data Extra).
(It also says you need to be living in NZ but i'm sure a trip to a Telecom NZ store will get you one).
SIM and MicroSIM Cards: Telecom Mobile
As for coverage, i'm not sure. TNZ has only just gone 3G in the past few years, they were all CDMA prior. If they've used existing towers and coverage is the same as their CDMA network, it'll be good, else Voda may be a better bet.
They're pretty much the same price, so take your pick. I'm 99% sure there's a Vodafone Store at AKL, so you could pickup on arrival.