Is it a requirement in NZ to be able to return to the airfield of departure in the event of a problem?
No but it's a requirement that you can land
somewhere.
When departing Queenstown non rnp you always have a plan A and a plan B (like most flights I guess) . What this guy did was compromise his PLan B.
eg Plan A, you climb to around about 5500ft VMC, at this stage, if you have two engines you have enough height to be able to lose one, go IMC, and still get out of the valley without hitting a hill. Plan B, get to 5400ft VMC, realise you can't make 5500ft without going IMC, descend and return into a visual figure of 8 pattern at the feild, and land. Part of plan B involves having enough daylight left to be able to depart, realise you can''t meet the requirements of the departure, go into the figure of 8 in order to align with the strip, and then conduct a visual landing. The 30 minutes is to allow you enough time to do that if Plan A doesn't work, and every now and then, Plan A doesn't work. Hope that helps explain it.