Oil tankers serving Australia's oil needs are vulnerable as they sail through the Straits of Hormuz, then the Straits of Malacca, then to and from the main refinery in Singapore. Click on image to make it even larger. Image courtesy of stratrisks.
One reason Australia maintains a submarine fleet is for early warning against threats to Australia's oil supply chain and to actively defend this chain.
Reported in Australia's Canberra Times, November 1, 2014:
"...two maritime hotspots that supply Australia with up to 70 per cent of its petrol, raising a concern over the nation's near-complete reliance on imported fuel.