The way I understand this if they are flying at 300m like they have done the last couple of days then they are within the convective mixed layer of the boundary layer. Within the surface layer, roughly 10% of the boundary layer, there is a log-log relationship of winds vs height, above this where it is well mixed they stay fairly constant. This then means a 70% reduction is needed to get to 10m winds.
If they flying in at 1,500m then they are getting towards the top of the boundary layer, but as this is so far south compared to here it will likely be in the boundary layer. At this level there wil be an effect from the entrainment heat flux. Above the inversion at the top of the boundary layer winds drops with with height and so if they are flying at 3km the reduction to surface winds will be less.
This may be wrong, but it makes sense to me.
