The Australian Government's forensic findings on the Double Six plane crash

According to the National Archives of Australia record series B5535:

Title of report: G. Bennett - Sabah Air Nomad - Report by Government Aircraft Factories (GAF) investigation team on a crash of Nomad aircraft in Malaysia 9M - ATZ on 6 June 1976.

Annexure 1S33(1)(a)

Reason for decision (not to make public the findings):

• Annex 13 of the Convention on International Civil Aviation - Aircraft Accident and Incident Investigation requires that a participating state in an investigation not release details of the investigation without the permission of the main investigating state.
• Malaysia has not as yet publicly released their final and full report of the investigation.
• The Australian Transport Safety Bureau undertakes investigation of aircraft accidents under Annex 13. The public disclosure of this information would lessen the confidence of foreign governments in Australia's commitment to meeting the requirement of Annex 13 and compromise the future activities of the ATSB and impair its ability to carry out its statutory function.
• Release of the information could reasonably be expected to cause damage to the international relation of the Commonwealth.

Annexure 1 S33 (1) (b)

• The information was of an inherently confidential nature when communicated to the Australian government.
• The information is still afforded security protection by the foreign government and it has asked that the information not be disclosed to the public.
• The disclosure of this information would therefore constitute a breach of confidence owed of that foreign government.

Images from the National Archives of Australia's website:

Front of sealed envelope

Back of sealed envelope

A letter from J.H Dolphin

The plane which crashed was an Australian manufactured GAF Nomad N-22B type twin engine turboprop passenger aircraft operated by Sabah Air, which looks similar to this:

Specs:

Crew: 1-2 pilots
Capacity: 12 passengers
Length: 12.56 m (41 ft 2⅜ in)
Wingspan: 16.52 m (54 ft 2¼ in)
Height: 5.52 m (18 ft 1½ in)
Wing area: 31.10 m² (324.0 sq ft)
Airfoil: NACA 23018 (modified)
Empty weight: 2,150 kg (4,730 lb)
Max. takeoff weight: 3,855 kg (8,480 lb)
Powerplant: 2 × Allison 250-B17C turboprop engines, 313 kW (420 shp) each
Cruise speed: 311 km/h (168 knots, 193 mph)
Stall speed: 88 km/h (47 knots, 55 mph) (power off, flaps down)
Range: 1,074 km (580 nm, 668 miles)
Service ceiling: 6,400 m (21,000 ft)
Rate of climb: 7.4 m/s (1,460 ft/min)

Government Aircraft Factories (GAF) was the name of an aircraft manufacturer owned by the Government of Australia based at Fishermans Bend, a suburb of Melbourne in Victoria. In 1987, GAF was reorganised and renamed as Aerospace Technologies of Australia (ASTA) then privatised. ASTA subsequently formed the nucleus of Boeing Australia.

Source
Daily Express, 26 February 2013