Sydney sweltered through one of its hottest days on record, with the city's west approaching a staggering 50 degrees on Saturday.

A southerly change brought gusts of 104 kilometres per hour on Saturday night.

But before that, the mercury in Penrith reached a scorching 48.9 degrees just after 3pm, making it one of the hottest places on earth and setting a new temperature record for the Sydney basin.

As the bushfire threat across large parts of NSW remained critical into Saturday afternoon ahead of predicted volatile winds, heat records tumbled and residents struggled to find ways to cool down.