1280px Nimbin when built
Nimbin under Danish flag on delivery voyage

The first Australian merchant ship lost during WW2 was North Coast Steam Navigation Co. motor ship Nimbin 1052/27 on passage Richmond River to Sydney which  struck a mine which had been laid by the German Raider Pinguin off Norah Head NSW  5 December 1940.
Pinguin Indian Ocean 1941

Nimbin sank almost immediately with seven out of 20 crew lost .

Passat was the ex Norwegian tanker Storstad which had been captured by Pinguin off North West Australia 7 October 1940 and fitted for mine laying working as an auxilary with the raider Pinguin.

Prior to Nimbin the British flag  Federal Steam Navigation steamer Cambridge 10,486/1916 on passage Melbourne to Sydney hit a mine off Wilson’s Promontory 7 November 1940 laid by the auxiliary Passat.
cambridge 40
The American Hog Island (former steamer which had been converted to a motor ship),City of Rayville 5,883/1923, American Pioneer Line bound for Melbourne suffered the same fate off Cape Otway the next day, hitting a mine 8 November 1940 laid by Passat.
city of rayville
City of Rayville
Interestingly City of Rayville was the first American Merchant ship sunk by enemy action in WW2
Source: Captain Iain Steverson