Well that is a possibility at any outdoor wedding. The chances of rain affecting your ceremony is relatively small.
But with most weddings on North Stradbroke a back up venue is always on the back burner. Generally the reception venue will be the option of first choice. Otherwise a home rented for the couples overnight accommodation can be used. Where ever the ceremony takes place it would run as organized at the day before’s run through.
Remember this is a unique island and weddings held indoors have a special magic. Every ceremony I have attended that had to use to a back up venue has been a moving, emotional experience.