The position of your mail box should be inside your property. If you are erecting on the verge, it must not obstruct the footpath.
NZ Post provides information about designing, setting up and positioning your mailbox.
NZ Post Mailbox specifications (nzpost.co.nz)
Before digging, please ensure you check for cables and pipes under the ground. BeforeUDig is a free online service which allows you to find the location of cables, pipes and other utilities.
BeforeUDig (beforeudig.co.nz)