Established Telcos don't allow this option even though they could (do for selected customers).
The NBN has its flaws but it does represent a changing of the guard and hopefully the competition will shake out some innovation.
Found out recently there's a new NBN supplier in Tasmania (only) that bills by the day. Only pay for the days you want it. Switch to speeds between 12Mps and 250Mbps as you want.
The cost of incumbancy. I'm not sure the established Telcos have accepted that suddenly they're back on a level playing field. Possibly they're just exploiting the change over period but the old adage of big is best is no longer an adage.