On 10 August 1977 Air New Zealand letter HO:B:22 requested authority to conduct five flights overflying Antarctica in the McMurdo area undertaking to operate these flights to the specification earlier submitted with the following exceptions:
“a. A proposal to permit descent to 6000 feet QNH in VMC or by the approved NDB procedure in IMC provided that:
1. Cloud base to be 7000 feet or better.
2. Visibility reported to be 20 kms or better.
3. ASR is available and used to monitor flight below flight level 160.
4. No snow showers in the area.
Flight in the McMurdo area below flight level 160 will be restricted to an arc corresponding to a bearing of 120° Grid through 360° G to 270G from the NDB within 20 nm in order to keep well clear of the Mr Erebus region."
The change was the requirement for ASR monitoring was altered to 'coordinated with local radar control' and the descent in VMC from F160 to A060.
To be honest, a descent in IMC to A070 following the original procedure was safer than the later amendment requiring visual terrain clearance - but then again only if it was actually followed too I guess.