From: Iowan in MD/DC
Don't bother with New Zealand - it's way too far, and you can accomplish the same blockade effect by simply taking enough of the Australian coastal bases that you can make sure nothing gets in (or out).
If you're going for Australia, I'd do it as early as possible. Before June, if you can. Even then it might be too late to take Sydney. That place is a real beast. I'd shoot down from Rabaul to Luganville, and get Townsville at the same time as Noumea. March west with a single armor unit to take Normanton and points north. March the rest of your stuff down the coast, using armor to flank west and control paths of retreat. Reevaluate your chances at Brisbane, before you trigger the reinforcements. At the same time, you should have landed at Geraldton or Esperance to take Perth, and then start marching along the rail line to Port Augusta. Once you have Noumea, Brisbane, and the islands off Sydney, he can't bring anything in that way. That leaves Melbourne. Park some float-plane cruisers/subs in the area and you'll at least see things coming in/out, and you can use naval air to interdict. From Albany (that's the name, right?) and Esperance, you can try to interdict with Netties or at least force him to go much farther south.
Make sure you don't skimp on naval search to Luzon in the interim, lest some xAKLs, or even xAKs, sneak by with supply. In this scenario, I envision taking Java and Palembang early, Mindanao as well, and living off of the fuel from Java/Balikpapan.
You'll likely have to forgo anything in the Gilberts area, and much of any consequence NE of Noumea. I would be shocked if you could also get Suva and still have time to get Australia.
The key to it all is his preparation and forts level. Any unit that can be cut off from retreating to Sydney should be cut off, otherwise you're looking at sizeable garrison. That's where the armor and aggression leapfrog landings down the coast come in.
I've tried this against the AI. Once Sydney is well stocked, taking it just isn't really possible.