The flag of the Australasian United Steam Navigation Company is a white saltire throughout, top and bottom 'quarters' red, hoist and fly blue. The Hunter's River Steam Navigation Company, started in the early 1840s, was taken over in 1851 by A.U.S.N., based in Sydney.
According to information obtained from the Nautical Association of Australia Inc. publication The Log, the originating company of the Hunter [or Hunter's] River Steam Navigation Co. liquidated and reformed in 1851 as the Australasian Steam Navigation Co. with a flag diagonally quartered red and blue as confirmed by Liverpool Chamber of Commerce Sheet of 1885, Reed 1891 and Griffin 1895. In 1886 the company was acquired by British India Steam Navigation Co. Ltd. and the following year it was amalgamated with the Queensland Steam Shipping Co. to form the Australasian United Steam Navigation Co. Ltd. The Queensland Steam Shipping Co. had been formed in 1887 by BISN with its flag being red with a white saltire which was the BI colours and basic design in reverse. The new flag thus combined the two. From what I can make out from various comments the origin of the original companies was aimed against overseas companies such as BISN being involved in Australian trades. So, they came in through the back door. AUSN, although continuing to be involved and regarded as an "Australian" company and although shown as based in Sydney by Reed 1901, was actually set up in London and was a British company and subsequent sources show this as the head office where as a BI subsidiary it ended up part of P&O and came to an end with the sale of its last ships in 1961.