A record summer in Sydney and a series of early-season heatwaves helped make 2017 the city's thirstiest year since 2004, Sydney Water said.

The results come as the city braces for another burst of heat this week, with the mercury set to climb to 37 degrees on Sunday. The searing temperatures will be even worse in the west, with Bureau of Meteorology forecasting 41 and 45 degrees in Penrith for Saturday and Sunday, respectively.

Claudia White, 4, cools off at the Wild Play Garden in Centennial Park. Credit:Jessica Hromas

Sydney's summer of 2016-17 was almost 3 degrees above average for mean temperatures, breaking a high mark set in 1990. This summer also got off to a warm start, with December expected to be among its four warmest in records going back to 1858.

Across the year, average daily water demand for Sydney was 1.587 billion litres, or 14 per cent higher than the 10-year average, Sydney Water said.