Saint Marys Bay had a population of 2,205 at the 2018 New Zealand census, a decrease of 81 people (-3.5%) since the 2013 census, and an increase of 36 people (1.7%) since the 2006 census. There were 978 households. There were 1,089 males and 1,116 females, giving a sex ratio of 0.98 males per female. The median age was 44.6 years, with 204 people (9.3%) aged under 15 years, 435 (19.7%) aged 15 to 29, 1,143 (51.8%) aged 30 to 64, and 420 (19.0%) aged 65 or older.