Russia Ukraine war, page-190004

The generally accepted norm in Geopolitics is that
a country can't bolster its national security at the
expensive of a neighbour; particularly if that neighbour is more powerful.

IMO that norm would apply equally to PNG with respect to Aus as it does
the Ukraine with respect to Russia.

And by the way, PNG, East Timor & Indonesia have all been invaded in the past.

https://journals.sagepub.com/doi/pdf/10.1177/0974928418821484

https://dppa.un.org/en/shanghai-cooperation-organization

Australia has decided to not be a member of either organisation
because its would be seen as compromising our National Security