Make sure you book in advance, we book through one place that does all the cruises and activites.
They don't increase prices and they help you work out times so nothing clashes.
We use BoatHireNB and found them on Facebook.
We booked the package to Port Stephens from the Visitors Centre in Sydney. Remember to browse through all the brochures of various agencies before u make a booking. The price differs for the same itinenary.