4G is now almost universally available across all three carriers, pretty much everywhere cellular service is available. 4G on 700 MHz is on band 28 like in Australia, Asia or Europe, not on bands 12-17 like in the US. If you plan on visiting very rural/remote areas and wish to make phone calls, make sure you have VoLTE (4G calling) enabled on your device, as some of these areas have LTE service only.

5G is now available in New Zealand, but networks are limited to dense urban centres such as central business districts, suburban centres and airports. One NZ, Spark and 2degrees all use the 3500 MHz band (n78). One NZ is currently the market leader in 5G coverage, followed by Spark and then 2degrees.
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Price comparison with the rest of the world
UK internet comparison site cable.co.uk has ranked New Zealand as 218th out of 237 countries/territories and 33rd in the OECD (36 countries) in terms of mobile data affordability with data costing on average NZ$10 (approximately US$5.89) per GB (August 2023). This means New Zealand's data prices are approximately US$4.24 per GB more expensive than the global median, and 13 times as expensive as neighbouring Australia. Amongst the OECD, only the US and Switzerland have more expensive data than New Zealand. [Cable 1]