Some of the guys asked about the current. It's not called hells gates for nothing
All the water from the "Wild Rivers" Franklin, Gordon, Collingwood etc etc run into Macqaurie Harbour and funnel through a very small entrance. One side is shallow and impassable and the other is deep water but the flow rips through there at an amazing speed. So you can imagine what it is like when you got 12m seas meeting that flow - lots of people have died
This image you can see Ocean Beach on the north side of Macquarie Heads and the shallows on that side. The deep water is on the southern between the island and the coast.