Marchwiel covers . It had an estimated population of as of with a population density of people per km2. Marchwiel had a population of 3,507 at the
2018 New Zealand census, an increase of 240 people (7.3%) since the
2013 census, and an increase of 276 people (8.5%) since the
2006 census. There were 1,383 households, comprising 1,734 males and 1,770 females, giving a sex ratio of 0.98 males per female, with 699 people (19.9%) aged under 15 years, 696 (19.8%) aged 15 to 29, 1,377 (39.3%) aged 30 to 64, and 735 (21.0%) aged 65 or older. Ethnicities were 88.4% European/
Pākehā, 11.1%
Māori, 3.2%
Pasifika, 4.6%
Asian, and 1.5% other ethnicities. People may identify with more than one ethnicity. The percentage of people born overseas was 11.9, compared with 27.1% nationally. Although some people chose not to answer the census's question about religious affiliation, 51.3% had no religion, 37.3% were
Christian, 0.5% had
Māori religious beliefs, 0.8% were
Hindu, 0.3% were
Muslim, 0.1% were
Buddhist and 1.5% had other religions. Of those at least 15 years old, 240 (8.5%) people had a bachelor's or higher degree, and 822 (29.3%) people had no formal qualifications. 231 people (8.2%) earned over $70,000 compared to 17.2% nationally. The employment status of those at least 15 was that 1,278 (45.5%) people were employed full-time, 345 (12.3%) were part-time, and 123 (4.4%) were unemployed. == Education ==