Your marriage should be registered in the country where it took place. There is no legal obligation for you to also have your overseas marriage registered in New Zealand. Generally a marriage solemnised abroad in a non-polygamous country in accordance with the laws of that country will be deemed valid in New Zealand. Such marriages are recognised in New Zealand for the purposes of residence visa or citizenship applications.
If you wish to register your marriage in New Zealand, the 1995 Marriages Act requires that the witnessing of marriages be conducted by a New Zealand representative. You must therefore have it witnessed by an official from the High Commission.
A fee will be charged for witnessing a marriage. Also, if there are any costs incurred such as accommodation and travel, these are to be met by the person who requires a witness.