Sydney Harbour, between Balls Head and Goat Island. 24 November 2016.
Vessel at 33°50'59.69" S 151°11'45.89" E

Data from https://en.wikipedia.org/wiki/HMAS_Protector_(ASR_241)

Name: Seahorse Horizon
Owner: Defence Maritime Services, Acquired: 1998, In service: 1998
Homeport: HMAS Creswell
Displacement: 670 tons full load, Length: 42.7 m (140 ft), Beam: 9.5 m (31 ft), Draught: 3 m (9.8 ft)
Propulsion: 2 Detroit 12V-92TA diesels; 2,440 hp (1.82 MW) sustained; 2 Heimdal cp propellors
Speed: 11.5 knots (21.3 km/h; 13.2 mph)
Range: 10,000 nautical miles (19,000 km; 12,000 mi) at 11 knots (20 km/h; 13 mph)
Endurance: 14 days
Complement: 6 civilian or 9 navy (for training)
Sensors and
processing systems: Navigation Radar: JRC 310 I-band. Decca RM 970BT I-band. Sonar:Klein side scan.

HMAS Protector (ASR 241) was a Royal Australian Navy (RAN) trials and submarine rescue ship. Built in 1984, the ship was initially operated by the National Safety Council of Australia as MV Blue Nabilla. She was purchased by the RAN in 1990 for use as a surveillance, training, and diving support vessel. During her military career, Protector supported the trials of the Collins-class submarines, and was involved in a search for the shipwreck of the World War II cruiser HMAS Sydney. The ship was decommissioned in 1998 and, while still owned by the Commonwealth, she is provided to Defence Maritime Services to allow them to support Navy activities under contract. Renamed Seahorse Horizon, the ship is operated by Defence Maritime Services out of HMAS Creswell as a training and Fleet support vessel

Further information from BR_Reef (thank you)
Designed by Phillip E Curran Pty Ltd and built as the offshore supply vessel BLUE NABILLA by Elder Prince Marine Services Pty Ltd, Fremantle to their own account and operated as such until sold 2/1986 to the National Safety Council of Australia - Victorian Division (see Wikipedia for background), it left Fremantle 1/3/1986 for Port Welshpool. Arrived 13/6/1989 at Melbourne.

Sold 8/1990 (for $3m) to the Government of the Commonwealth of Australia, Department of Defence, Royal Australian Navy, arrived 18/10/1990 at Sydney; formally commissioned 30/10/1990 at Sydney Fleet Base as the trials and safety vessel HMAS PROTECTOR (ASR241). Management transferred 1998 to Defence Maritime Services Pty Ltd, and the housing rebuilt at Southern Maritime Shiplift, Launceston; 1/7/1998 renamed SEAHORSE HORIZON.