A chart showing the location of Great Detached Reef (bottom right) and Thursday Island (top left)	Great Detached Reef - Port Arthur is located at the south-eastern corner of GDR Lagoon and is marked

Great Detached Reef is a reversed C shaped reef made up of a number of separate sections. It is located a little off what is the main outside section of the Great Barrier Reef, although here that reef is merely a series of small bommies or reefs. Great Detached Reef is 11.5 nm from north to south and 8.8 nm east to west. The west (open) side of the reef has a series of bommies and small reefs along a large part of its length.

There are only a couple of boats that travel to this area, one being Kalinda which routinely does full boat charters there in November and December. Another boat does some trips there and one more might go there once or twice a year.

Kelly and I travelled here in November 2016 on Kalinda with fellow members of St George Scuba Club when we chartered the whole boat. We flew into Horn Island (next to Thursday) and started our trip from there.

The Great Detached Reef has a huge lagoon. Port Arthur is located on the inside of the lagoon down towards the south-eastern corner. Its location is GPS S11º 47' 57.4" E144º 04' 17.8". The bottom is a very gradually sloping sandy/coral mix with lots of small and larger coral bommies. The depth is only 2.5 metres or so next to the reef and about 12 metres where the boat anchors. It is excellent for night dives.

The name Port Arthur was given to it as there are supposed to be large numbers of convict surgeonfish (for non-Aussies, Port Arthur was the penal colony in Tasmania). However, we never saw one.

A banded pipefish	Two more banded pipefish, heads top right

We anchored in about 15 metres and we dropped to the bottom off the side of the boat and then headed south-east towards the shallower area. We dropped to nine metres and immediately saw the first of many small coral bommies. They range in size from 2 metres across to 5 metres. Most only come up a metre or two from the bottom, but some are a bit higher.

As soon as we went to the first bommie we found a pipefish. These are banded pipefish (Corythoichthys intestinalis) I think. In some books another species was also called banded pipefish. Anyway, they were everywhere! I counted 19 on our day dive but none at all on the night dive. Looks like they must all hide during the night.

I think this is a barrier reef anemonefish	A green turtle we saw on the night dive

We then went from bommie to bommie to the south, and then headed north back around a very large bommie that creates a canyon between the main reef and the bommie. The sand here is only 2.5 metres deep or so. We went around this and then worked our way south to the end of the bommie and then west back to the boat.

I saw a few small sharks, some saw a leaf fish (lucky!) and plenty of small tropicals. There were plenty of surgeonfish, parrotfish and trevallies. I also saw a few lionfish. On the night dive I had a large green turtle come from out of nowhere and swim right up to me.

A photo of the divers on the night dive	Another shot from the night dive

There were also lots of different species of shrimp on the night dive, but they were impossible to photograph. There was also a parrotfish in a coocoon and a couple of silver-tipped reef sharks, one of which was quite large.

We dived here in November, the water temperature was about 28ºC, visibility varied from 10 to over 20 metres (on the night dive). A really good dive site.