AirNZ checkin

Hi All. I am travelling from Sydney to AKL via AirNZ but booked through a 3rd party app. AirNZ max checkin allowance you can pay with your ticket is 23kg, however we are trvaelling from SG to SYD with 30kg. If I add a prepaid baggage, does it mean I have to add another bag? And not for the 7kg excess or my 1 checkin bag?