I think you have been mislead. The Australian Government, however upset they might have seemed over a couple of Australian citizens being sentenced to death for drug trafficking, certainly would not have even contemplated an armed conflict over the matter. The comments the Prime Minister made would have been done simply to make it clear to his own people - the Australian populace - that he wasn't happy with such a severe sentence for a crime which, in Australia, would have probably merited no more than a few years in gaol.
It would take an act of military aggression against Australians living in Australia or those nations with which it is allied, I expect, to persuade Australian to declare war on another country. Australia has experienced many wars, great and small. It knows the price in human suffering a war causes.
As far as the average man in the street in Australia; there was a great deal of sympathy, even anger, directed towards the Indonesian legal system and those that supported such a drastic sentence as 'shooting by firing squad.' This is because this country abolished the death penalty decades ago. But as previously stated, you wouldn't get Australians willing to fight people they have nothing against personally, just because of the way Indonesia treats its drug offenders.