It is never revealed who the invading army are; but they appear to be invading due to the need for land.
It turns out that the origin of the invading army is never revealed. It is suggested that the original author did this in order to limit offending the country in real life, due to the way in which the army acts in the actual series. In my research, the enemies are shown to be (mostly) cruel and 'with no redeeming qualities'. I have found speculation that the army is French, Chinese, Malaysian, Korean, Indonesian, and more generically, Asian.
A constant motive I find is that the invading army wishes to take advantage of all of our undeveloped space. I guess no-body told them that it is undeveloped because it is also, mostly, uninhabitable.
We have more information on who they are not.
An article on rsimpson.id.au provides insight into what we know of the enemy, from the books:
The Enemy at rsimpson.id.au(emphasis mine):
Anyway, it is easier to identify who they are not. They are not:
- New Zealand (as they fight on our side)
- Papua New Guinea (as the fight on our side)
- The United States of America (as they supply us with equipment)
- Japan (as they supply us with equipment)
- India (as they try and broker a peace deal)
So who are they ? What are their characteristics ?
- They speak a guttural language, with a sort of wailing music to it (The Dead of the Night, p44)
- Their language is nothing that Fi recognises (and she knows about 6), Lee doesn't recognise it and he knows Thai and Vietnamese, Homer does not recognise it and he knows Greek nor does Ellie (with her poor French).
- They are ruled by a general - likely a military dictatorship (*Tomorrow, When the War Began*, p168)
- They have a large population, several times ours (otherwise they wouldn't have a hope of pulling of an occupation)
- They use a different alphabet to us [Australians](The Night is for Hunting, p128 - a soap packet wrapper)
- They were either colonised by a European power with a royal family or have one of their own (The Other Side of Dawn, p192 - Colonel Long's rank insignia is three crowns) but not by the British as the invader's insignia for a Colonel is a crown and two stars.
- They have at least one aircraft carrier (The Third Day, The Frost, p82)
- They have a base nearby to Victoria (that's where they invade, and for a successful invasion you need a nearby base, see Invading Australia
The author of the article goes on to suggest that what we know does not match any single country, and that they believe this to have been a deliberate plot device, on part of John Marsden. The author makes note of a popular theory that the army is simply Asian, but comments that they interpret the army as French.
There is suggestion that the invaders are a South Asian coalition.
The Wikipedia article on the series comments that the invading army is a coalition of South Asian countries, seeking a solution to over-population.
"Tomorrow series" at Wikipedia (emphasis mine)
Likewise, no parts of the war outside Ellie's immediate perspective are covered; the reader is not informed exactly how much of the country is under enemy control, or how well the war is going for the Australian military. However the invaders are described in the series as a coalition of highly populous South Asian nations seeing the vast underdeveloped spaces of Australia as a way to tackle their overpopulation (Most likely Indonesia, Malaysia and the Philippines) and the series features multinational colonies of enemy civilians being set up in conquered territory.
Another user finds a lot of similarities with Indonesia.
A user has also asked about the army, on Quora, and once again, we are only met with speculation:
"In John Marsden's Tomorrow, when the war began, who is the invading army" at Quora(emphasis mine)
Beren Wollensack, Lives in Australia, misses Harambe
Updated Feb 10 2016
"It is never actually mentioned, in the books or movies. It is likely to be Indoneisa, our neighbour to the North has a strong military, and they have a lot of peopl[sic] and not enough space. They aren't as fortunate as Australia for sure, and in the movie, it is mentioned that the invaders are Australia's "less fortunate neighbours[sic]."
It can't be China, simply because then New Zealand, even in with their military super boosted by the war, would never be able to win against China. Apparently the North Korean flag can be seen on the helmets of the soldiers in the movie, and it makes sense, nobody from North Korea will ever be offended by this, and I didn't notice this flag at all, keeping in mind I learnt[sic] this fact after watching the movie. The invading army seems far two equipped and capable, with tw[sic] is, army, and good aircraft, to be the DPRK. And even then, the U.S. Would probably jump at the chance to save a nation from invasion from North Korea, especially a western country, and an ally.
I can't see it being China or North Korea, so even though technically, the answer is nothing, because it is never mentioned, but I would have to say Indonesia, Papua New Guinea was allied with Australia during this war, and they also wouldn't be able to successfully take over Australia. The same with Malaysia, but they don't have a very high population, so they wouldn't need to invade as much as indoneisa[sic] would. I guess it could be Singapore, they have a strong airforce, but because they probably don't have a good enough population for the invasion, and they could easily be beaten by Indonesia, even though I think Singapore has a better airforce, if they were to invade Australia, assuming indoneisa[sic] doesn't help."