Sydneysiders responded to the near-record heat with the highest water use in 15 years, with another bout of heat likely to build by the end of the week.

Unstable conditions after another scorcher on Monday prompted the Bureau of Meteorology to issue a severe thunderstorm warning for Sydney and a strip of along most of inland coastal NSW from near Taree down to the Victorian border.

The city's top on Monday of 31.9 degrees was again eclipsed by severe heat in the western suburbs, with 43.6 degrees in Richmond.

Penrith backed up Sunday's 47.3 degrees – the hottest in the Sydney Basin in 79 years and the warmest for the day on Earth – with 42.5 degrees on Monday.