The threat was not real, the Government knew it but didn't tell the public. Japan's objective was to prevent US using Australia as a military to stage attacks on their shipping lane from the south, the bombing of Darwin was specifically targeting US naval assets. And by the way 1939 was when Germany started war on Europe, Japan already started invading China by 1931.
"Across the years, history books and high school lessons have repeated the stories of a Japanese invasion plan, foiled only by the diggers' desperate efforts on the Kokoda Trail and the United States' naval victory in the Coral Sea. An imaginary "Brisbane Line" was drawn to represent Australia's second line of defence against the approaching hordes.
The trouble is, someone forgot to tell the Japanese. The only real invasion plan appears to have existed in the minds of the Australian public.
Japan never seriously intended to invade Australia, a fact known to the Australian Government by mid-1942 and confirmed by intelligence reports.
There was no Japanese invasion plan before 1942 and that Australia barely rated a mention in the 1941 conferences which planned Japan's strategy. The plans got no further than some acrimonious discussions. The army dismissed the idea as "gibberish", knowing that troops sent further south would weaken Japan in China and in Manchuria against a Soviet threat. Not only did the Japanese army condemn the plan, but the navy general staff also deprecated it, unable to spare the million tonnes of shipping the invasion would have consumed."
