Go to https://www.skinny.co.nz/topup , top-up $5 via debit/credit card. No need to contact Skinny, Keep the existing direct debit going and don't change it. When the $5 runs out or a year passes, repeat top-up.
Be thankful you are not with Kogan where there is literally no way to sent a txt to a shortcode/premium/non-standard number. They have no account balance feature at all so no way to debit your account the charge, you are just fully out of luck.