When design work began in November 1940, the AC1 was originally intended to be a 2 pounder gun-equipped vehicle, that was intended to be a true Cruiser tank, with a weight of between 16 and 20 tonnes. Due to a lack of home grown experience in tank design a mission was sent to the US to examine the M3 design and Colonel W.D. Watson MC, an artillery officer with many years tank design experience was provided by the UK. He arrived in December 1940. Like the Canadian Ram, the Australian Cruiser was to be based on the engine, drive train, and lower hull of the American M3 Medium tank, mated to an upper hull and turret built closely along the lines of a British Crusader. By 1942, attempting to keep pace with German tank developments, the design specification had become more like an American medium tank, resulting in a heavier design and a higher silhouette profile.
The Australian Cruiser tank Mark 1 (AC1) was designated "Sentinel" in February 1942. Manufactured by the New South Wales Railway Company, fabrication took place at Sydney's Chullora Tank Assembly Shops with serial production vehicles emerging in August 1942, the premises also being used as a testing ground. The design used existing parts where available from other tank designs, simplified where necessary to match the machining capacity present in Australia. The hull was cast as a single piece, as was the turret; a technique not used on the hull of any other tanks of the era.
The original vehicle was designed to mount a QF 2 pounder this was later changed to a QF 6 pdr (57 mm, 2.25 in). However, none of these were available and the first 65 tanks were built with the 2 pounder. Two Vickers machine guns were carried as secondary armament, one in the hull and a second mounted coaxially beside the main gun. The preferred engines suitable to power a 28 tonne tank, a Pratt & Whitney Wasp single row petrol radial, or a Guiberson diesel radial, were not available within Australia, so the Sentinel was powered by the combined output of three Cadillac 346 in³ (5.7 L) V8 petrol car engines installed in clover-leaf configuration (two engines side-by-side to the front and a single to the rear: all three feeding a common gearbox). Sixty-five production vehicles had been completed by June 1943.
The Sentinel was to be succeeded by the AC3, a much improved design with better armour protection, and most importantly increased firepower. The next step up in firepower available in Australia was the 25 pounder (87.6 mm, 3.45 in) gun-howitzer. This was quickly redesigned as a tank gun, work that would later prove useful for the design of the Short 25 Pounder. Mounted in a fully traversable turret larger than that of the AC1 but using the same 54 in (1.4 m) turret ring, it was slightly cramped for the turret crew but gave the AC3 both armour piercing capability as well as an effective high explosive round. The hull machine gun and gunner were removed from the design to make room for stowage of the larger 25 pounder ammunition. Powered by the same three Cadillac V8 engines as the AC1, they were now mounted radially on a common crank case and geared together to form the "Perrier-Cadillac", a single 17.1 L, 24 cylinder engine, very similar in some respects to the later A57 Chrysler multibank used in some variants of the US M3 and M4 tanks. One pilot model AC3 had been completed and work had started on producing 25 tanks for trials when the programme was terminated.
In an effort to further improve the firepower of the Australian produced tanks, a turret was developed and mounted on one of the earlier development vehicles to assess the vehicle's ability to mount the foremost Allied anti-tank gun of the day – the British 17 pounder (76 mm, 3 in). This was achieved by mounting two 25 pounder gun-howitzers which when fired together would significantly exceed the recoil of a 17 pounder. It was later fitted with a 17 pounder and after successful gunnery trials the 17 pounder was selected for the AC4 design. For the AC4 the 17 pounder was to be mounted in a new and larger turret, attached by a 70 inch (1778 mm) diameter turret ring, the space for which was accommodated by changes to the upper hull permitted by the compact nature of the "Perrier-Cadillac".
The completed Sentinel tanks were used for evaluation purposes only and were not issued to operational armoured units. The Australian Cruiser tank programme was terminated in July 1943 believing it better for Australia to put the effort spent on the AC tanks towards building her own railway locomotives and supporting the large number of US tanks due to arrive. The tanks that had been produced were placed in storage until the end of the war. In 1943, the 3rd Army Tank Battalion was equipped with a squadron of AC1 tanks which had been modified to resemble German tanks. These tanks were used in the filming of the movie The Rats of Tobruk. This appears to have been the only time a squadron of Sentinels was used for any purpose.