I know that many people have mentioned the same problem, that interim visas were coming to an end when they hadn't had any response about their application within the six months' validity. When interim visas were first introduced, this situation had not been envisaged, and was never supposed to happen. Some people have written to INZ pointing out their situation, which was difficult BECAUSE of INZ's own delays, and have found that their case was quickly taken up and dealt with.
See here
https://www.rnz.co.nz/news/national/...rs-with-delays where this situation came into public knowledge.
In your place, I would press INZ for THEM to give YOU a solution. If you remove yourselves from the country voluntarily, your case stops being so urgent.