Witherlea had a population of 5,409 at the 2018 New Zealand census, an increase of 576 people (11.9%) since the 2013 census, and an increase of 846 people (18.5%) since the 2006 census. There were 2,115 households, comprising 2,631 males and 2,775 females, giving a sex ratio of 0.95 males per female, with 987 people (18.2%) aged under 15 years, 741 (13.7%) aged 15 to 29, 2,409 (44.5%) aged 30 to 64, and 1,269 (23.5%) aged 65 or older.
Ethnicities were 90.0% European/Pākehā, 10.0% Māori, 2.1% Pasifika, 4.0% Asian, and 2.8% other ethnicities. People may identify with more than one ethnicity.
The percentage of people born overseas was 17.8, compared with 27.1% nationally.
Although some people chose not to answer the census's question about religious affiliation, 50.4% had no religion, 38.5% were Christian, 0.4% had Māori religious beliefs, 0.6% were Hindu, 0.2% were Muslim, 0.5% were Buddhist and 1.7% had other religions.
Of those at least 15 years old, 786 (17.8%) people had a bachelor's or higher degree, and 831 (18.8%) people had no formal qualifications. 714 people (16.1%) earned over $70,000 compared to 17.2% nationally. The employment status of those at least 15 was that 2,172 (49.1%) people were employed full-time, 708 (16.0%) were part-time, and 81 (1.8%) were unemployed.[7]