This document describes a probabilistic forecasting model for long-term peak electricity demand in South Australia. The model uses 15 years of half-hourly electricity demand and temperature data, as well as economic and demographic data, to forecast peak demand 20 years into the future. It is a semi-parametric additive model that accounts for calendar effects, temperature effects, and annual trends related to GDP, price, heating and cooling degree days. The model generates probabilistic forecasts to capture the uncertainty in long-term peak demand predictions.