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It is useful to see the Customs information page on Electronic device examinations. It is somewhat easier to read than the actual text of the law linked from your question.
If Customs suspects, under "reasonable suspicion", that a traveller is involved in criminal offending, then they may request access to search your phone. One of two things might happen here if you refuse:
There is no provision for imprisonment just for refusing a phone search. The worst that can happen is that you can be refused entry to New Zealand, and returned home at your expense. (This could happen for many other possible reasons, too.)
Of particular note is:
Penalty for not giving access
Only a New Zealand Court can impose a penalty of up to $5,000 on a traveller who does not give Customs access to an electronic device. This is only possible if Customs decides to prosecute the traveller.
There is no $5000 "instant fine" for denying access to search a phone. The fine would only be imposed after a court hearing.