I always liked the fact that the invaders were unnamed. Considering when the book was written to when this film will be released it has allowed Marsden’s work to be relevant to changing politics. ‘Naming’ things in a work of fiction always dates them- which can be great in adult fiction but my teenage students hate reading anything that feels dated to them.

i know of at least two countries with the capacity to invade Australia in this scale and they would be North Korea as where 1 in 6 people are in the army airforce or navy and they have a total population of 23,479,088 (July 2008 est.)divide that by 6 and you have an army of 3,913,181 which would easily be able to invade australia espically when in the first book it clearly states that Australia is not really expecting it and not ready for it due to commeration day. the actual standings of the north korean army are as standing:
The Reserve Military Training Unit consist of approximately 1.7 million persons (men 17-45 and unmarried women 17-30) who are not either in active duty or important rear area personnel. They are mobilized under supervision of provincial military units, for a total of forty days’ training out of the year.
* The Worker-Peasant Militia is a combination of older men aged 45-60, along with men ages 17-45 and unmarried women ages 17-30 who are not included in Reserve Military Training Unit. They train for a total of thirty days out of the year. Their current numbers stand at 4.1 million.
* The Young Red Guards consist of 1.2 million male and female Higher Middle (High) School students aged 14-16. They are subject to a mandatory four-hour drill session every Saturday and a total of 160 hours of on-campus drills annually. A total of 450 hours of off-campus training is also mandatory.

another very likely country that it could be is of course China. and these are their millaraty thingies:
it is estimated at about 7.5 Million troops,18,000 tanks and 8 AK47s. Never underestimate the power of stupid! From all the various sources (non classified) if they went to war, on day one they’d have 2.7 million in all services. By D+30 they’d decide how many of their 30 million reserves they want to activate. If I remember my history correctly, the chinese have been stockpiling weapons since WW2. When they funneled weapons to Vietnam from Russia, they kept a third. So the militia divisions raised would have an assorted collection of weapons from the last 40 years with a very few modern weapons. The chinese do have something like 350 submarines, 6 of which are hard to find with modern technology. They have destroyers and frigates, 100′s of missile boats, and an odd selection of non-mission specific naval vessels which are defenseless. Chinese heavy industry, power generation, and war making capabilities are centrally concentrated in 1960′s era facilities (result of their russian cousins teaching them communism) which are easily eliminated. But the country is so large you could expend all 22000 nukes and they would still have 600 million people. If is hasn’t escaped people’s notice, every country in the world has a chinese community deeply entrenched in the R&D community. The USA couldn’t get ahead without it’s core group of chinese scientists. The chinese could put up 18,000 jet planes and flood out/taiwans defenses. If they were smart (who says they’re not), they have stockpiled 10,000 tactical missiles to saturate our defenses at sea and on land. But why? They are a patient culture and can wait another 1000 years for the US, Russia, France, UK, and others to sell them all the weapons and technology they’d need. With 4 trillion in US securities, they could destroy our economy in a day. India has fought them three times and vietnam once in the last 20-30 years and all have lost. India lost over 8000 soldiers. Vietnam 1500 and retreated. It didn’t matter to the chinese how many soldiers they lost. Those tactics haven’t changed in the chinese army. Besides some specialty regiments, the mass of chinese soldiers are still fighting the 1950′s war.

of course their is also Indonesia that could pull it off plus the are so extremely close to Australia to do it

hope this helps you out Nick