Your "jet stream" is lower than you realise. More likely the surface wind.
First clue - weather map. Bookmark
http://www.bom.gov.au/australia/char...ptic_col.shtml because seeing is usually best when there is a big, fat high pressure system - labelled "H" - with the centre smack over your location.
The weather systems move west to east and a high-pressure cell west of Perth will take 2-3 days to reach the east coast.
Second clue: Isobars (lines on this map) close together = lots of wind = poor seeing. Isobars far apart = calm, and (maybe) good seeing. A cold front (curve with pointy bits) means seeing will be utter crap.
And guess what - the isobars are quite tight AND there's a front coming. The next high-pressure system for you is at least 2-3 days away (its west of Perth).
Lastly, I don't suppose you have noticed this yet, but there is a seasonal pattern to the latitude of high-pressure cells - in winter the centres tend to pass over Sydney's latitude (well north), while in summer they tend to pass further south (Melbourne/Hobart).