We got Elliott Bonnett (Cheif Network Architect at Chorus) to provide a response.
Upstream and downstream multiplexing uses two different techniques to share the common fibre communication channel. In the downstream direction each ONT is addressed individually. Frames are transmitted on the PON from the OLT and are received at all ONTs. Only the ONT the information is addressed to passes this to the end user, all other ONTs drop the information (this functions much as early multiaccess Ethernet configurations do). In the upstream direction a TDM protocol is used, where timeslots are allocated to each ONT as required, and only one ONT then transmits at any one time.
Initially Chorus' most common optical split ratio was 1:32 for the initial couple of years of deployment. The ITU GPON standard supports up to 1:128, but the physical limitation is an optical budget of 28dB.
We moved to 1:16 in conjunction with an Architectural change which also saw us move from housing splitters in above ground cabinets to smaller below ground fibre closures.
We also use a range of split ratios, down as low as 1:2 in rural, to increase reach. Each 1:2 split reduces power levels by 3dB. At an average loss of 0.4dB per km, each halving in the reduction in the split ratio gives us around 7 to 8km of extra reach in rural.