Since trackwork took place on Saturday, the vehicle positions feed seems to not contain any vehicles south of Waterfall.
Thanks.
Looking at this in a bit more detail, it seems that trains also disappear north of Wyong.
Previously, Non Timetabled traffic such as freight didn’t show in these areas as they are outside ATRICS territory, however since the weekend even passenger trains don’t show in these regions.
It seems that over the weekend, the supplemental contributing feeds to the sydneytrains vehicle position API went offline, and only the ATRICS feed remains, which covers the Sydney metro area, but not beyond Waterfall, Wyong and Katoomba(though of course the Blue Mountains line is closed at the moment).
For the Hunter and Southern Highlands lines (and the Kiama to Bomaderry section of the South Coast line), data on the position of diesel passenger trains is available in the nswtrains feed, so as far as I can tell, this outage effects strictly Waterfall–Kiama, Wyong–Newcastle Interchange and Katoomba–Lithgow.
Seeing as it was working well up until last weekend, I’m guessing that it must just be that the process which integrates non-ATRICS data into the feed was disabled or crashed and wasn’t re-enabled.
Looking forward to seeing this working again as my home station of Thirroul is in the effected area 
Happy to report that the issue seems to have been resolved.