Well in regards to PNG, the referendum held clearly showed that the locals wanted independence. So it then makes it hard for Australia to ignore the wishes of the locals, especially if the UN are on our backs. And the relationship that Australia had with PNG was nothing like the one the USA had with Hawaii or Alaska.
The next thing, and probably far more important, is the fact that PNG would have cost Australia far more than what Australia would have gotten back in return. Although that may be true of all the smaller states, it'll still add up & cost significantly more than subsidising Tasmania, West Australia, and to a lessor degree South Australia & Queensland.
Plus I have little faith in the fact that the locals in PNG would instantly behave themselves just because they're a state of Australia. More than likely that'll even reinforce their efforts at local independence & make any such insurrection even worse than the OTL. And this would take place in the aftermath of Australia's involvement in Vietnam. Furthermore, as cranos pointed out, there's the business with Indonesia/PNG/East Timor. IMHO it would more trouble than it is worth. Better give them their independence & let them stuff it all up, rather than we stuff it all up for them.