From the early 1900's Maori had been "mainstreamed" to follow the pakeha practice of using Surnames and to not swap them around as we did in the century prior.
However, many whanau still swap between a Pakeha surname and an transliterated version of the name in Maori, for example, Wirihana / Wilson.
Within many Maori whanau are also nicknames that a person is known by all their life. How many of you have an Aunty Buba or an Uncle Boy?
Also, many Maori born before about the 1960's also have a Maori and transliterated first name, for example, Mere / Mary. These days, many people are also taking a Maori name in addition to their pakeha name, and not necessarily a transliteration.
Another practice is for a person to be known by their second name, or middle name.
While many whanau are all related, be aware that if someone has the same Maori whanau name as yourself, they may not be your relations, for example there is a Herewini whanau from north and a Herewini whanau from the east coast, as well as a Selwyn whanau from Dargaville. As far as the writer is aware, these whanau are not related through the Herewini / Selwyn lines.
Another thing to be aware of is that a name like Te Ingoa is not necessarily the same as Ingoa by itself and are often two very different whanau. When looking for names like Te Ingoa, in the NZ White pages it will be under Te as is right, however, there are some lists that file the name under, in this case, I for Ingoa. The Kinder library is an example of this, but after discussion, they are correcting this and will update their site to reflect the corrections.