Update. Heard comms today on 261.925MHz, around 0330UTC.
Aircraft callsign missed, but heard voice "calling in the blind, looking for 0330 Christchurch weather".
Followed a short time later by a data transmission lasting about 20 seconds. These occurred every half hour, so I assume they were Christchurch weather reports in text.
Finally heard part of aircraft voice comms, but response not heard, which suggests Base Ops may have been operating direct and not via SATCOM?.
Aircraft may be using HF or CPDLC to talk to MAC CENTER.
At 0630UTC heard same type of data transmission on 263.725MHz, the frequency they have used in previous years for voice comms.
Anyone recognise the data mode and/or suggest a program that might be able to decode?