There aren't frequency boundaries as such within high level sectors. Larger sectors own a number of frequencies & how they're managed is up to the controller. Generally you use the nearest outlet & there are customary transfer points (acts as a memory prompt) but there are no actual defined areas. It's about as strictly defined as waving at a chart & saying "you use frequency X here, Y up there & Z down here". And yes, when it's quiet sectors are combined to cover geographically huge areas with many frequencies, plus they're also doing the G airspace underneath.
This page has PDFs of all sectors & indicates the rough geographical area covered by each frequency. If you look at the Alice Springs chart you'll see the A airspace frequencies listed with an "A". For instance 123.9 is at Oodnadatta, 131.8 at Ayers Rock & 128.85 at Alice.
NOTAM Maps | Airservices