It's the down and dirty time of the year in the southern hemisphere. Generally speaking, once the temperatures drop trout would rather starve than rise in the water column to grab a bite. This is even more true of fish running from lakes into rivers, especially if the water is deeper. There are always exceptions and certainly fish will chase a fly a long way laterally across a river to eat, but in order to entice a bite we need to get our nymphs in the faces of the trout, and to do that effectively we need to sink them fast.
Winter nymphing means putting your nymphs right in the zone.