You are correct about dual gauge but only south to North East. The south to NW is only standard gauge from the points where you have placed the note.
The rest of the lines here are broad gauge only apart from the very top EW lines - both SG & BG
The right hand curve was built in the 1990s as part of the Melbourne- Adelaide railway and then the left hand SG curve in 2000's 

This allows trains from Northern Victoria to bypass Tottenham rail yards to head south towards Brooklyn

Thanks to Marcus Wong and a few others... 
See https://www.railgeelong.com/gallery/newport-sunshine/tottenham-triangle/F129_7942.jpg.html