You need to contact Airbnb and cancel the booking so guests can get refunded. Extenuating circumstances. Should you not, the guest can contact them and get the refund anyway, as seen in other post further down.
I am sorry for you all in NZ! You will have a busy time ahead getting those roads going so things can return to normal. You have a beautiful country when it is standing still.
just checking: have you heard from your guests? Maybe they are nearby already. Message them so there are no surprises!