Upvote:1
Although it's too late to help in this specific case, the following information will be useful for any other British citizens in a similar situation. (I'm guessing this case refers to a British citizen, since the asker refers to their mother returning to London prior to a planned visit to New Zealand.)
In the event that a passport is stolen, a British citizen can obtain an emergency travel document if there is insufficient time to obtain a replacement passport before planned travels. For details, see https://www.gov.uk/emergency-travel-document