John Marsden wrote six novels in the series. Frequently taught in Junior High school.
The premise begins with typical Aussie teens who live in the rural hinterland of one of our cities out having fun in the bush when some unknown and unnamed invader sneak attacks and pretty much overwhelms Australia's small but high tech military. Traditional Allies such as the US/UK/EU and Japan kinda sit back and do the bare minimum they can to help, leaving the brunt of the war to the New Zealanders, Australian forces in exile and resistance fighters.
Generally well written despite the fact for six novels No specific country or race of invaders is mentioned. Taps into a deep seated almost irrational fear that something like this will happen one day.
Reckon this one will get pegged on Communist China. Only Plausible potential enemy Australia has.
Would be enough to scare off any major troop/combat deployment by the western alliance/Japan.