The Minmi Trench is a system of gullies on the northern side of the Botany Bay entrance, named for the coal that litters its sand and boulder gaps – almost certainly cargo from the collier SS Minmi, which was wrecked on Cape Banks on 13 May 1937, though some may be from the SS Woniora lost 2.5 km to the south-east. The reef top sits at 16–20 m and the main wall runs north-west to south-east, dropping to 22–23 m with sand extending to 26 m on the southern side. Between the wall and a line of large boulders lie three separate sections of trench, deeper towards the south, with numerous swim-throughs, overhangs and small caves concentrated south of the anchorage; to the north-east is a very large, low cave or overhang.

The sponge life is outstanding – reds, blues, greens, yellows and whites – with sea squirts and gorgonians crowding the trench walls, and it is a productive site for macro. Fish and other life include eastern blue devilfish in the caves, red morwong, angelsharks and Port Jackson sharks on the sand, cuttlefish and octopus in the cracks, large schools of silver sweep and yellowtail over the reef, and black reef and mosaic leatherjackets. Because it sits in the bay entrance, tide governs the dive: on the ebb, particularly after rain, dirty water pours out over it, while on a high or incoming tide visibility of 12–20 m and more is normal.

The site is large enough that even with a scooter McFadyen has covered only 500 m of it, and he considers it needs several dives. No charter boats currently visit, so it is a private-boat or St George Scuba Club dive, an easy 15 minutes from the Botany Bay ramps and a natural partner to Henry Head, Pink Floyd or the Cape Banks Caverns.

Boat only. From the Botany Bay ramps (Cooks River at Kyeemagh, Foreshore Road, or Bonna Point at Kurnell) run to the northern side of the entrance between Henry Head and Cape Banks, about 1 km apart; the site is roughly midway, 10–15 minutes from Kyeemagh. Anchor at the GPS mark where the sounder jumps from 25 m to about 20 m and then 16 m.

The old visual marks (the Kurnell water tank and the 'golf tee' radar tower) are gone – the tower was demolished in 2025 – so use GPS. No charter boats currently dive it.

Sheltered from southerly swell by the Kurnell peninsula opposite but open to east and north-east swell rolling into the bay entrance; keep it for seas under 1.5 m. Being at the mouth of Botany Bay the tide matters more than the wind: on the ebb, especially after rain, dirty bay water flows out over the reef and visibility can collapse, so dive on a high or incoming tide when 12–20 m+ is normal. Tidal current through the entrance can be noticeable at the 22–26 m level.

Container ships and tankers use the channel to the south – stay near the anchor line and use an SMB.

From the anchor at 16–20 m head south-west along the main trench for 100 m+ through the swim-throughs, overhangs and small caves, drop over the wall to the sand at 23–26 m briefly, then come back along the reef top. On a second dive go north-east to the very large low cave. Scooters cover 500 m+; several dives are needed to see it all.

No regular charter – private boat or dive-club (St George Scuba Club) only