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The NZ government has passed laws that state who can do plumbing. The reason that no-one will answer your questions is because it is not legal for you to do your own plumbing. Plumbers do make make the laws but like the public, they are subject to huge fines for breaking the law. The following is from the Plumbers Gasfitters and Drainlayers Board

What is sanitary plumbing?
Sanitary plumbing is any work involved in fixing or unfixing any pipe, plumbing fixture or appliance including; any trap, waste or soil pipe, ventilation pipe, or overflow pipe and any pipe that supplies or is intended to supply water.

All sanitary plumbing must comply with the Building Code and, where a building consent is required; the work must be checked by the building inspector from the building control authority in your area (your local council). A code compliance certificate cannot be issued until the work has been signed off by the building inspector.

A certifying plumber is responsible for the testing, verification and the supervision of licensed plumbers, limited certificate (trainee plumbers) and exempted persons.

Sanitary plumbing does not include the installation of appliances such as dishwashers and washing machines; the replacement or repair of taps, ball valves and plugs.[/i]

Who can undertake sanitary plumbing?

Authorisation type   
Licence category   Certifying plumber
This is the highest qualification available. These people are responsible for ensuring both their own work, and the work of anyone they supervise is done competently.

Licensed plumber
These people are qualified and licensed but must be supervised by a certifying person who is ultimately responsible for ensuring the work is done competently.

Limited certificate trainee plumber
These are people who are working towards becoming qualified. They can do the work but a certifying person must ensure it is done competently.
Exemption for plumbing under supervision   Exemptions
These people are not registered and don't have a full qualification, but they can do plumbing work provided they are supervised by a certifying person, who must ensure that the work is done competently.

By your reasoning, as long as I murder someone on private property, the laws of the land dont apply. I'm sure that the judicial system here would disagree

i don't know what any of that has to do with taking measurements.

so i how do i figure out which one i need to buy so i can get a plummer to install it?

Ifmyou import a cheap,product,mwhat guarantee do you have that ot will work?  If you buy one in NZ from a plumbing merchant, Mitre10 or Bunnings, the onus is on them to honour warranties. The public seem to think that products purchased locally are too expensive, but every plumber can tell you of cheap chinese crapmthat didnt work. I would not recommend importing a product.