You don't need a building consent.

According to the information you have provided, your retaining wall will not require a building consent under the following exemptions of Schedule 1 of the Building Act 2004:

• Exemption 20

Please note

• Construction cannot start until all required resource consents have been obtained.
• If you are building over or close to a public wastewater or drinking water drain, you will need an approval from Watercare or Veolia.
• If you are building over or close to a public stormwater drain, you will need an engineering approval from Council. Call us for more information on 09 301 0101.
• Check any other enactments or regulations (for example: Unit Titles Act, Land Transfer Act, Property Law Act, Hazardous Substances and New Organisms Act, Health and Safety at Work Act, Heritage New Zealand Pouhere Taonga Act, Auckland Council Bylaws etc.) and property rights that may affect the proposed work. It is the responsibility of the owner to check these prior to starting your project.
• Make sure your proposed project is built to the New Zealand Building Code.