Earthing and earth stakes.
ANSWER
New Zealand uses a "Multiple Earthed Neutral" (MEN) system. This system requires the main Neutral to be earthed at the main switchboard of each installation (home or building).
The earth is required to provide safety to appliances used in the home. When these appliances are faulty they will may short out to the steel casing. If the steel casing is not earthed it can potentially electrocute the next person to pick it up.
The main earth connects to the incoming neutral of the mains cable and then, by a large green wire, to an earth stake or earth electrode.
The Earth Stake or Electrode is a steel or copper pole driven into the ground and is accessible outside your home. The green cable connects to this via a brass clamp which is often covered over by a plastic in-ground lid. The wire will also have a copper or plastic "Earthing Conductor Do Not Disconnect" sign on it, it is important that this is present.
See also Earth Fault Loop Impedance Tests
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