Can you measure the inside distance across the two machined faces and the diameter of the pin? There is a possibility of it being either the Tank Vickers MG (low likelihood), or either the .303 or 7.62mm versions of the BREN.
L4A4 Bren (7.62mm conversion) was used by Armoured Corps in Aus (but not Inf), although this would seem an unlikely choice for a co-ax MG.
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