This paper presents approximate estimates of the size, growth, and changing demographic characteristics of Australia's lesbian, gay, and bisexual (LGB+) population. We used weighted data from a longitudinal panel survey at three time points to estimate the population by sex and age group in 2012, 2016, and 2020, and we applied cohort analysis to help understand the drivers of its growth. The LGB+ population aged 15 and above doubled in size over the eight-year period, increasing from 0.58 million in 2012 to 1.18 million in 2020. More men than women identified as LGB+ in 2012, but this had reversed by 2016. Our estimates show that the LGB+ population is young and becoming younger over time, with cohort analysis showing that its growth has been driven primarily by recent cohorts of young women with a bisexual identity. We discuss what might be driving this growth and whether our results suggest continued rapid LGB+ population growth in future.