It is usually applied automatically to your Azure subscription.
So, in your scenario, the system will automatically apply the $139 NZD credit, and you will be charged the remaining $61 NZD.
The difference should be payed from your credit card attached to subscription.
If you want to further clarify you can open a billing support request free here.
https://azure.microsoft.com/en-us/support/create-ticket
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Regards,
Abiola