People living with HIV are entitled to an active, healthy sex life. You do not have to tell a sexual partner that you have HIV, as long as you take appropriate precautions to prevent sexually transmitting HIV.
In New Zealand, what is considered 'appropriate precautions' is using a condom for vaginal or anal sex. While adhering to treatment so you maintain an undetectable viral load offers more protection than condom use, undetectable has yet to be recognised under the NZ law.
Therefore you DO NOT have a legal duty to disclose your HIV status before having vaginal or anal sex if you use a condom, however if you do not intend to use a condom, you DO have a legal duty to disclose your HIV status before having vaginal or anal sex without a condom (regardless of your viral load).