The basics...

In New Zealand there is no specific law proscribing nakedness in public. There are two sections of the Summary Offences Act 1981 to consider. First - obscene/indecent exposure (S27) and second - offensive behaviour and - disorderly behaviour (S4). 

The first S27, has been accepted as inappropriate for mere nudity because a lewd or lascivious element must be proved. Under S4, an appeal against a conviction of offensive behaviour has been allowed by the High Court, on the grounds that intent to offend (forensically) has to be proved. A conviction for disorderly behaviour, though, has been upheld by the High Court, in a case where the nakedness was in a suburban street. (Not a place where the defence of it being: 'not uncommon' or 'known to occur,' applied.) It is worth noting that the police's chief prosecutor in the Office of the Commissioner accepts that where the nakedness 'is known to occur' or 'is not uncommon,' it can be "said to be not illegal." (Unfortunately that hasn't stopped North Shore City writing a bylaw trying to say it is!) 

In other words, having your gear off in the street is likely to get you a $100-$200 fine (if you either get a 'legitimate' complaint laid against you, or you ignore a policeman's warning). Otherwise, on a beach where 'everyone knows about it' you're fairly safe. 

The bad news...

Having said that though, there are still a few caveats lurking... In the British system of justice, the court represents the power of the King - essentially, it is absolute - so a judge can do just as he pleases! That includes ignoring decisions of his peers and seniors; and exactly that has happened.
In Waihi, in 2000, a Naturist was charged with disorderly behaviour for being naked at the 'place where it's known' on Waihi Beach.

The District Court Judge, though admitting he knew of the 1991 High Court precedent acquitting a Naturist on the charge of offensive behaviour, actually changed the original charge to offensive behaviour before finding the defendant guilty of it! 

One assumes that judge didn't personally approve of skinny-dipping and was quite happy to risk the opprobrium of his peers to make his point. There is little doubt, the defendant's lawyer said, that the conviction would have been overturned on appeal - but in the end, the defendant decided; who wants to spend another $2,000 to prove - what?

In practice though, at present in New Zealand the likelihood of legal action is very low. 

The good news... 

In 1990 New Zealand enacted a statute to affirm recognition of the International Covenant on Civil and Political Rights, and the Universal Declaration of Human Rights; which both declare the human person to have inherent dignity and worth. That statute is the Bill of Rights Act. 

That 'dignity and worth' obviously cannot be selectively applied to chosen bits - it must apply to the complete human person - if it is not the whole person, it is meaningless. 

Therefore no part of the being which has inherent dignity and worth, can be rationally determined to be an attack on society. No mere part of a human form can rationally 'offend' or 'disorder' the average reasonable person - as conflated with the Bill of Rights. If that average reasonable person, that is: the Bill of Rights Act itself; is not offended or disordered, any reaction that is inconsistent with that now defined norm, must be forensically unreasonable and thus disregarded. 

Any volunteers to let the police and courts know...?