This Site is dedicated to all those families of the people that have tragically disappeared on flights in and around New Zealand. I  only hope that from all the effort in building this site and from all the effort of those taking part in this venture, that it will bear fruit in bringing ‘closure’ to their memories!
Gavin Grimmer

If he left as per the time given by  the Aeroclub member at West Melton, and the Halkett resident (as “12 ish”), and so taken as 12.00pm:

100 knots = 2.12pm
110 knots = 1.59pm<
120 knots = 1.53pm
130 knots = 1.41pm

Taking into account the general pattern of the weather that day appeared to be predominantly north-westerly  I would say at a guess, the ground speed would have been somewhere around the 100 to 110 knot mark.
The majority of observations of this plane seen/heard in the Haast area was in the 2.00pm range, so this calculates pretty well.

Going by the information given in the reports from Haast - Jackson Bay - Mussel Point - up to a report given by Barbara Dibben (just north of Haast), we can again calculate that FMQ traveled in the region of 46 n.miles, which added to the times above:

100 knots = 2.39pm
110 knots = 2.24pm<
120 knots = 2.16pm
130 knots = 2.02pm

Given that Barbara Dibben gave a time of 2.30pm when she saw it, still shows that the 100 to 110 knots range is pretty good.
He would have been flying into a head wind heading south of Haast and hence would have had a lower ground speed, but because he turned around and headed north again, this would have given him a tailwind, and as the distances are roughly equal, it time would balance out - slower down/faster back.