If Australia was asked to take a greater role in the defence of Malaya, not only has it the problem of defending Malaya/Singapore, but as you suggest, must find the forces to ensure a reliable, safe supply route remains open from Australia, through the Dutch East Indies. The area, in the main, is best defended by strong Naval and Air forces. Both, Australia had troubles expanding, and the big Naval units can only come from Britain and the USA. It did have a chain of airfields, and sent troops to defend the airfield near Kupang, Timor, first stage on the trip along the island chain towards Java.