Simply put, the airline placed their crews in a situation where errors were more likely to occur. They simply weren't trained for what they were being asked to do
That statement, whilst correct, is not the whole story.
Air New Zealand and NZALPA went to some lengths to ensure that their senior pilots and members were seen as professionals, who knew it all and did not therefor need to ask advice from elsewhere, such as the RNZAF < USAF< USN or the division, wrote Bob Thomson in his History of New Zealand Antarctic research programme 1965-88
Nor did Air New Zealand take advantage of the experience gained by members of their aircrews who flew earlier flights. Apparently the NZALPA saw the Antarctic flights as a "special perk" for their members and had an agreement with Air New Zealand that flight crews should be spread widely amongst its members,
Therefor aircrews, including aircraft captains, usually had not any previous experience on these flights, an experience which would have avoided the Erebus disaster from ever happening..
Bob Thomson was not a pilot, but probably had done more flights to the Antarctic than anybody, usually in the cockpit, and at times was the commentator for Air New Zealand scenic flights to the ice.