When you say "disused" do you mean "not used very frequently" , or completely unused?
Nadzab was totally disused from around 1945 to the mid 1970s, when the Australian Government constructed the current airstrip and taxi ways.
Rumour had it that the Australian Government wanted Nadzab as a forward defence airstrip and required the PNG Government to close Lae and use Nadzab for all commercial flights and move the PNG Defense Force Air Wing to Nadzab, to justify the expense.
Until the mid to late 1970s, all operators used Lae airport, including the DC6B and later Lockeed Electra (and possibly the B727 I don't remember) from Sydney and Brisbane landed at Port Moresby and Lae, before heading south again.
Nadzab was disused until Air Niugini was forced back there in the late 1970s. The regional air services and air charter survived at Lae a little longer but eventually all operators were forced to Nadzab and Lae airstrip closed.
Olderairhead may remember more details?