Page 16 of the manual: “An island is a territory located entirely inside one sea zone”
Since New Guinea’s territories are located within 2 sea zones, it is not an island for game proposes and planes can’t be scrambled from airbases on them.
You can place airbases and naval bases anywhere. If any of New Guinea’s territories had an IPC value of 2 or more you could also build an IC there since it’s isn’t an island for game proposes.
New Zealand is considered an islands (pg. 3 of the Official FAQ).