Oz military no longer sell any topo charts to Joe Public. Don't quote me, but it may be due to liability issues (no regular amendment cycle, Joe Public doesn't have access to amendment data).
Under ICAO, each State is responsible for producing their own WACs. PNG CAA stopped doing so sometime between independance and now.
NMB were importing the TPC etc from Oz when they had access to them.
Pilots going to PNG CAA for charts were directed to NMB. When NMB stopped getting Oz mil charts, they told the pilots "NOGAT". No explanation why (maybe NMB didn't know why). Word didn't get back to CAA that NMB weren't getting the Oz mil charts, so nothing was done about the problem for a while. I don't know what the state of play is now, but I believe that aerial mapping was commenced under a foreign aid program in about 2007 with a view to restarting the production of WAC.
The answer to the problem is for PNG CAA to start producing WAC in accordance with their international obligations, be that via their own means or international assistance.