There are three souces for population forecasting data:
forecast.id.com.au – this site provides LGA level forecasts based on analysis conducted with Council input, combined with Census data. Available for most (but not all) LGAs.
https://www.planning.vic.gov.au/land-use-and-population-research/victoria-in-future-2016 – Victorian Government forecasting based on Census data, planning data and information provided by each Council. This is available for all LGAs.
Both of the above currently use 2011 Census data as a base, however future updates should use 2016 data.
https://digital.atlas.gov.au/datasets/019e1fed4e9f429eaf9189a0a461ead5_1/explore?location=-35.833906%2C144.817975%2C7.19 – ABS quarterly Estimated Residential Population – current figures use 2021 Census as a base.