Operational statement 19/04A1 sets out the commissioner's statement on using a kilometre rate for business deductions of a running motor vehicle - deductions.
Income Tax Act 2007 Section DE 2B, Election to use kilometre rate method or costs method2 in Subpart DE—Motor vehicle expenditure, prescribes the election of either a kilometre rate method or a cost method for apportioning the use of a motor vehicle when it's partly used for business and other purposes. If no election is done, the cost method is assumed. Once a method is elected it cannot be revoked.
Section DE6 - DE11 deals with using a logbook.
2.https://www.legislation.govt.nz/act/public/2007/0097/latest/DLM7271117.html