deadhead,
did the briefing instruct the pilots on how they were to determine their azimuth from the TACAN?
As they should have still been above MSA, 16,000ft, until they were in the designated let down area, then the AINS would have been quite sufficient to determine azimuth from the TACAN CH 29. As it was a mandatory VMC descent then if they were not positively VMC, ie positive fix from known ground features, then the descent should not have continued below MSA.
I cannot find the reference right now, but it is my belief that this descent procedure was arrived at after consultation with Deep Freeze ops and NZCAA.