Flinders Islet is the island Wollongong locals call Toothbrush Island because of its long thin handle and lumpy 'brush' end, and it is one of the five low islands of the Five Islands Nature Reserve that shelter the approaches to Port Kembla. Its northern end is cut by a series of gutters that run west-south-west to east-north-east across the reef, giving one of the more interesting shallow boat dives in the Wollongong area.

The dive starts in a large gutter about three metres wide with a rocky, kelp-covered bottom around 12–14 m. Swimming east you pass into a sequence of narrower channels – roughly two metres at the mouth and closing to barely a metre at one point – with walls 10–16 m tall that are dotted with small sponges and encrusting life. The gutters lead to the cave, which undercuts the island at about 10 m. The entrance is a few metres high and opens into a chamber that runs back under the island, with a long arm branching right for another 15–20 m. Small gorgonians grow inside, red rock cods sit in the gloom, and the floor is fine silt, so good finning technique matters.

Outside, the kelp-lined gutter floors are good places to look for weedy sea dragons, and striped seapike patrol the channels. Visibility around the islands is frequently better than on the mainland beaches. The site is shallow enough for Open Water divers who are comfortable in a confined space and with a torch, and the gutters offer interest even if the cave is skipped. It combines well with a deeper dive at Toothpaste on the southern end of the same island, or with Pig Island further out.

Launch from the Port Kembla boat ramp in the Outer Harbour (behind the steelworks; large car and trailer park, toilets). Wollongong Harbour's ramp (Belmore Basin) is closer to the northern islands but suits only small boats in calm seas. From Port Kembla steer out through the harbour entrance, keeping clear of the shipping channel, and head for the island; the run is 10–15 minutes in a small boat.

Head north-east to the distinctive toothbrush-shaped Flinders Islet and run to its northern end. The cave entrance mark is 34° 27.271' S 150° 55.806' E; anchor on the reef at 12–14 m just west of the gutters. Wollongong Harbour ramp is an alternative for small boats in calm conditions.

The Five Islands sit in open water off Port Kembla and are exposed to all swell, but the islands themselves give lee on one side or the other: dive the northern ends in southerly swell and the southern ends in north-easterly conditions. Boats generally want less than about 1.5–2 m of swell and winds under 15 knots for the short run out of Port Kembla Outer Harbour; Wollongong Harbour's ramp is suitable only for small boats in calm seas. Visibility is often better than the inshore beaches, but drops after heavy rain when Lake Illawarra, Allans Creek and the harbour flush brown water out.

Best visibility is usually late autumn to early winter; winter westerlies produce flat seas. The northern end of Flinders Islet is best in southerly swell; north-easterlies push swell and surge into the gutters.

Enter the large (about 3 m wide) western gutter and follow it up to about 10 m, then work east through progressively narrower parallel gutters – one narrows to about a metre – until you reach the cave mouth. Explore the main chamber and its right-hand arm with a torch, keeping off the silty floor. Exit and follow the eastern wall north, then drift back west through the outer gutters to the anchor.

United Divers (Fairy Meadow, Wollongong) runs boat dives to the Five Islands from Wollongong Harbour/Port Kembla when numbers and weather allow; Expedition Dive (Sydney) runs seasonal seal trips (June–Oct/Nov); otherwise private boats and clubs.