Depends on when the mid life upgrade occurs if it occurs. If its all finished by say 1965 then by Mid 70s everything wpuld start to be wearing out again, not just the drive train but everything else in the tanks. Like all the electrical stuff. Keep in mind the 1st centurion arrived in australia in 1952 and they exited serivice in 1977 thats 25 yrs of service. So a mid life upgraded would have been needed in the 60s anyway
The deliveries of Centurions are as follows.
• 60 Mk.3 medium tanks purchased in 1949 and delivered from September 1951 and were still being delivered after August 1952 were subsequently upgraded to Mk. 5 standard between 1957 and 1961. (ARN 169000 - 169059)
• 51 MK. 5 medium tanks purchased in late 1954 and delivered during 1955 - 56. (ARN 169060 - 169110)
• 4 Armoured Recovery Vehicles (ARV) Mk. 2 ordered in late 1954 and delivered in 1955 - 56. (ARN 169111 - 169114)
• 6 Mk. 5 medium tanks delivered in 1956- 57 (ARN 169115 - 169120)
• 4 Bridge Layer Mk. 5 delivered in early 1960’s, the first arriving in September 1961. (ARN 115541 - 115544)
• 8 Mk5 and Mk5/1 medium tanks were purchased from New Zealand during 1968. The medium tanks were cannibalised for spares, and while the ARV was taken into service, it only served in Australia. (ARN 169123) A small number of New Zealand hulls were used to replace mine damaged hulls and at least one tank, ARN 169017, returned to South Vietnam for a second tour fitted with a New Zealand hull.
• 15 Mk5/1 medium tanks purchased from UK stock held in Hong Kong arriving in 1972 (ARN 169124 - 169138).
Vietnam gave the fleet a good shake up. A total of 58 Australian Centurion tanks (various variants including gun tanks, dozers, bridgelayers, etc.) served in South Vietnam, 42 were damaged, six assessed as beyond repair. Those that did not get damaged were sent back to Australia after 2500 track miles. So by the early 70s it would be difficult to say X or Y tank was old and therefore worn out.
The big issue with the Centurion by the early 70 isn't really their condition but their overall capability. 1970 is when Israel started replacing the Metor and manual transmission in their Centurions, and they were still in active service in the early 80s. Australia's Centurions need the 105mm gun, new diesel/automatic drivetrain and fire control, or to be replaced.