As of the 2023 census, Selwyn District is the fastest-growing territorial authority in New Zealand, with a population increase of 29% over the prior five years.

The first inhabitants of the area were the Māori who first settled New Zealand from the Cook and Society Islands about 700 years ago.

The predominant Māori tribe today, in Selwyn and most of the rest of the South Island, is Ngāi Tahu, whose local marae (meeting house) is at Taumutu near the exit of Lake Ellesmere (Te Waihora).

In the late 19th century, European (chiefly British) colonists arrived and carved the area up into farmland.

Recent years have seen above-average population growth, making it the fastest-growing local authority in New Zealand in 2013.

Most of Selwyn's new residents have moved from Christchurch to settle on small "lifestyle" farms and in Selwyn's small towns which are within easy commuting distance of the city (e.g. Rolleston, Prebbleton, Lincoln, West Melton, Kirwee).

The 10 councillors are elected from four wards on a multi-member First Past the Post basis; the total number of votes a candidate receives are considered: a.)

Geographical features: Selwyn District contains within it two distinct regions: the plains and the high country.

The plains, where most of the population lives and the majority of activity takes place, form an expanse of low-lying, flat, and comparatively dry grassland.

The extreme south-east is dominated by Lake Ellesmere / Te Waihora, an expanse of water surrounded by marshes, formed by the out-flow of the Selwyn River / Waikirikiri.

The high country is a sparsely-populated region, mainly consisting of hill and mountain ranges and narrow river valleys.

The Southern Alps are responsible for the relatively low rainfall, and also lead to a foehn wind, the "Canterbury Nor'Wester".

This strong, hot and dry wind is most common in spring and summer, and on occasion reaches damage-causing strength.

The results were 84.1% European (Pākehā); 9.0% Māori; 2.4% Pasifika; 11.4% Asian; 1.5% Middle Eastern, Latin American and African New Zealanders (MELAA); and 3.0% other, which includes people giving their ethnicity as "New Zealander".

The Selwyn District Council owns 11% shareholding in Orion, the local electricity distribution company.