Not their greatest offer but an offer, nonetheless. 

SMALL 1.5Gb per 30 days (now $112 for 365 day plan) - Roughly $9.30/month
MEDIUM 4.0Gb per 30 days (now $175 for 365 day plan) - Roughly $14.58/month
LARGE 15.0Gb per 30 days (now $231 for 365 day plan) - Roughly $19.25/month

Credit: Cheapies

Last Kogan deal posted on Geekzone: Dec 2022?
(update: looks like last promo on Geekzone was in January 2023, posted in a different thread)

Valid for new and existing Kogan Mobile customers. Personal use only. Offer available on Kogan Mobile Prepay SMALL, MEDIUM and LARGE 365 Day plans purchased before 11:59pm NZST 05/04/2023 unless sold out prior or extended. Not available for commercial or resale purposes. Vouchers expire at 11:59pm NZDT on 31/05/2023.Activation or recharge must be made before the voucher expiry date. Only one voucher can be used per activation or recharge. Vouchers do not stack. Vouchers cannot be used in conjunction with any other promotional offers. Vouchers are not transferable for cash. The full amount of each Plan must be paid upon activation, recharge or purchase of a voucher. A Kogan Mobile SIM card must be ordered from https://www.kogan.com/nz/ or https://www.dicksmith.co.nz/ before a Prepaid Plan can be activated. SIM delivery takes up to 10 working days, after which you can activate your Plan.