The SS Commonwealth is a small wooden steamer wreck that has an unusual place in Terrigal diving history. Built in 1901, she was a 36.7 m, 188-ton wooden cargo steamer, and on 19 August 1916 she foundered off Terrigal while carrying coal from Lake Macquarie to Botany Bay, according to Muddy Puddle Diver's wreck notes. COAST Magazine places her at Foggy Reef off Bateau Bay and lists her as an active dive site.

Her main legacy for local divers is indirect. Muddy Puddle Diver records that Foggy Cave - now the centrepiece of Terrigal diving and a well-known grey nurse shark aggregation site - was discovered by Les and Fran Graham in the 1970s while they were searching for the wreck of the Commonwealth. The wreck itself is typical of a wooden steamer after a century on the seabed.

The timber hull has largely gone, leaving the durable machinery. The Marine Life Network notes two boilers, two engines and lots of copper rivets visible - so the machinery stands as the main structure with a scatter of fastenings and fittings in the sand around it. The Terrigal Underwater Group lists the depth at 42 m, the Marine Life Network at 37 m.

As with other Terrigal deep sites, the machinery attracts fish: yellowtail and bullseyes around the boilers, wobbegongs among the debris, and the possibility of grey nurse sharks from the nearby Foggy Reef aggregation during warmer months. Sponges and small gorgonians grow on the boilers. At 37-42 m the dive involves decompression for most profiles and is for trained deep or technical divers.

It appeals to those interested in local history and in the Foggy Reef system - and it is the wreck that led to Terrigal's best reef dive.

All boat dives in this area launch from the Terrigal Haven boat ramp at the southern end of Terrigal Beach, tucked behind The Skillion headland. From the Central Coast Highway follow Terrigal Drive into Terrigal, then Kurrawyba Avenue and Terrigal Esplanade to The Haven, where there is a paid car park beside the ramp; Pro Dive Central Coast's shop is a short walk away and gear is loaded on the ramp or the adjacent beach. Private boats need a ramp with good parking early on weekends as the car park fills by mid-morning in summer.

The wreck lies off Foggy Reef, near Foggy Cave off Bateau Bay, about 12-15 minutes north of The Haven. There is no published public mark; operators and Terrigal Underwater Group members hold the position.

Terrigal's offshore reefs are open to the Tasman Sea and are dived only when swell is below roughly 1.5 m; a north-east sea breeze building through the afternoon in summer and strong southerlies after a front are the usual reasons trips are cancelled, so morning departures are the norm. The Haven boat ramp is sheltered by The Skillion but the run out past the headland can be lumpy in a south-easterly. Visibility is best in late summer and autumn when the East Australian Current pushes warm blue water inshore, and worst after heavy rain when Terrigal Lagoon, Wamberal Lagoon and Avoca Lagoon open and stain the inshore water.

Winter water is cooler (around 15-17 °C) but often clear. Deep wrecks also need little current: with the East Australian Current running, lines can stream out and a drifting ascent in open water becomes likely, so operators often wait for neaps or a quiet current day.

Descend the shot to the boilers, then work around the two engines and the scattered hull area on the sand. Keep the dive compact and return to the shot within the planned bottom time for the ascent and stops.

Nitrox for shallower end or trimix per team standards; decompression gas; redundant gas. Protected historic shipwreck.

Deep wreck charters are run on demand by Pro Dive Central Coast and Central Coast Divers (Terrigal) for suitably qualified divers, and by Terrigal Underwater Group and technical diving groups from private boats; there is no daily schedule, so book well ahead and expect minimum numbers and qualification checks.