An experience I had in that area in 1989 convinced me that some funny things can happen when the S.E Trade Winds meet the Roaring Forties. In my Vari-Eze @ 2000ft with a TAS 135 knots the ground speed was 180knots as we went past Sale.
As we turned north at Wilson's Promotory the aircraft suddenly adopted the angle of a canoe going over a waterfall.
It then just fell for 300ft. I remember thinking that this was going too be bad when the falling stopped but as it turned out we just flew smoothly out of the bottom of the "waterfall " of air albeit 300 ft lower.Apart from needing a new pair of jocks when we got to YSCB no harm was done.
Some of the Meteorology experts on this thread might be able to explain exactly what caused the 30-45 degreenose down attitude to occur.
Cheers RA