From a 2002 edition of Design Standards for Licensed Aerodromes. Who knows what the standards were when Essendon was built. Standards current at the time of construction are grandfathered into the MOS (subject to some conditions)
Table 7–9: Runway Strip Width for Precision Approach Runways
........Aerodrome facility reference code 3,,4 Overall runway strip width 300m
7.17.7 – Where it is not practicable to provide the full runway strip width, a lesser graded only strip width not less than 90m for code 1 and 2 and 150m for code 3 and 4 respectively may be provided subject to landing minima adjustments..
No need for a safety case, just provide what you can to at least the minimum standard, and adjust your landing minimums.
CASA threw all the safety case stuff in later when they hardened up the language to "must" and "shall". The interesting part is why CASA said to put 300m in the docs, when it probably never had been.