See OZTAM have now officially dropped the idea of a "non ratings season". Practically probably makes little difference to 10 - frankly all three commercial networks seem to run out of money by mid-October.
I don't really know much about how it works but I feel like they wouldn't be announcing Big Brother while MAFS is running. Since that show seems to suck all the oxygen around commercial TV in Australia.
Given how utterly bad Survivor has performed in the first week, I'll bet Big Brother is looking more and more attractive to Ten right now. They need tentpoles that draw audiences, and Big Brother's numbers are looking even better by comparison.