There's bound to be some indications of Indonesia's intentions beforehand, so we should be able to justify pushing the reserve call up back about 6 months. Perhaps the official explanation, at least the one given to Indonesia anyway, was that it was a response to Australia's UN obligations in Korea, or to help out in Cyprus letting the British got to war in Europe. A bit thin, but aren't most political statements?
The timeline is important here too. 3rd Brigade probably goes over to Korea first and the reserves are called up at the same time to begin training. Recruiting efforts kick into overdrive and
maybe conscription sugar coated as a way of reducing unemployment, kick starting the economy or something like that.
Officially the reserves are only supposed to serve inside Australia as a defence only force, somewhat like the WWII militia were supposed to, however once Indonesia makes it's move, 1st (less 1 Armoured Regiment aka Koalas - protected species not allowed outside Australia
) and 7th Brigades are sent into action, 9th Brigade is sent to relieve 3rd in Korea who are brought back home for predeployment training and reaclimatisation for PNG and to give commanders an airborne option (3 Para battalion).
8th, 11th and 13th Brigades are deployed to the north of the country while 4th and 5th Brigades (plus the Koalas) are kept as "strategic reserve" but sent into disaster relief duties when the nukes hit.
3rd Brigade may not make it to PNG but could be redirected as a "fire brigade" at home.