The visa issue in PNG is confusing and I understand the confusion of the OP.
PNG used to offer visas on arrival for single entry business, multiple entry business and tourism. Not entirely sure of the date (2013 or early 2014) Mr O'Neil decided that AU was being obstinate in not allowing PNG nationals a visa on arrival into Australia so he canned visas on arrival for all visa types for Australian citizens travelling into PNG. That was a mistake, as tourism suffered and PNG is attempting to attract more tourists (and cashed-up Aussies is a no-brainer of a target), so visa on arrival was reintroduced ... but only for the tourist visa, not business visas which still have to be obtained prior to travel.
PNG does however, still permit citizens of many countries (NZ included) to obtain a visa on arrival for tourism and business.
As other have said, the most important rule in PNG is to expect the unexpected. My first trip here was on a multiple entry business visa on arrival which I did phone up and check how much it would cost a few days before travel. It was K250. Between then and my arrival, the Gov't decided they'd like more revenue and bumped it up 100% to K500. When I arrived I tried to tell the young lass that I had rung and checked and she just shrugged and pointed at the ATM. I went to the ATM and it didn't work (nothing unusual). I had K400 on me in cash and some AU$'s as well (not much) so I went to the money exchange booth and luckily they accepted coins and I was just able to get the K500 required for entry basically by emptying out the contents of my wallet.
I fly in or out of PNG basically every fortnight and I know the customs guy in HGU pretty well as a result, even to point of conversation about his grandkids (he picks them up from school and is often late to the airport) and his golf game (which is also often the cause of a delay in customs clearance), yet every time he studies my visa and stamps officiously!
I should also say that in PNG, nothing happens without a form filled out in triplicate and filed with at least 15 different departments. If anyone is familiar with "Hitchhikers Guide to the Galaxy" you may remember the Vogons and their penchant for filling out forms ... I'm sure Douglas Adams based them on PNG!
PNG, Land of the Unexpected!