The largest military vessel, yes... largest ship overall? Not by a long ways.
HMAS Success (a modified version of the French Durance class multi-product replenishment oiler) was 157.2 metres (516 ft) in length, with a beam of 21.2 metres (70 ft), and a draught of 8.6 metres (28 ft), with a full load displacement of 18,221 tonnes.
The Evans Deakin shipyard (at Kangaroo Point in Brisbane) constructed 81 ships between 1940 and its closure in 1976. The company built the largest ship ever made on the Brisbane River, the oil tanker Robert Miller (37,675 gross tons, 67059 dwt. Lbd: 785' x 108' x 43'. (239.3 x 32.3 metres), completed in 1974.
The Whyalla Shipbuilding and Engineering Works (at Whyalla, SA, at the northwest end of Spencer Gulf) was constructed in the winter of 1939-40, and built 66 ships before its closing in 1978. The oil tanker Amanda Miller (66,800 tons 780' x 109' x 43') was the largest ship built by Whyalla, completed in 1971.
Those two sister tankers were the largest ships built in Australia from the keel up to date.