I used constantly to confuse these two major Australian varieties until I focussed obsessively on structure and texture of the wines. Although there are innumerable different styles of each in this huge country, I have found the following general tips helpful to think about when trying to distinguish between the two.
Texture: Australian Shiraz has a plush texture. It’s velvety smooth, and on the mid palate almost feels like you’re surfing a wave of fruit. By contrast, Cabernet texture is grainier, both in terms of tannins and in terms of fruit.
Tannins: You don’t often hear it said, but Syrah as a variety has some of the finest quality tannins out there, texturally. Australian examples are great exemplars: there’s a velvety texture to the tannins which underscores the velvety quality to the fruit. Cabernet meanwhile is far more fine grained and very slightly woody.
Shape of the wine on the palate: Cabernet from Australia (as everywhere else) is typically linear and directional - it’s really going somewhere. Shiraz is round and full with a focus on the tongue rather than the edges of the mouth (as with Cabernet). This contributes to the effect that you’re sinking into a big cushion of fruit when you’re tasting Shiraz.
Other features: Cabernet is still often made as a medium bodied variety across Australia, and especially in Western Australia. Of course there are bigger scale examples in south eastern Australia, but increasingly, these are becoming the exception. Shiraz, by contrast, remains a full bodied variety in most regions of Australia, unless very conscious winemaking decisions have been taken to the contrary. Cabernet can be gently leafy (not herbaceous), especially from Western Australia, while many Shiraz still have the classic blueberry pie notes. Eucalyptus can be found in almost any red variety in Australia so does not help you.