South-eastern Australia is in for another burst of summer later this week, with most of coastal NSW, including Sydney, facing severe heatwave conditions.

A broad trough will direct heat from the Red Centre, pushing temperatures in western Sydney potentially close to, or exceeding, 40 degrees for five days in a row from Thursday.

Penrith - which this month posted 47.3 degrees, the hottest recorded in the Sydney Basin since 1939 - is forecast to bake during the spell.

Revised predictions by the Bureau of Meteorology indicate the region is likely to endure tops of 42 or 43 degrees on Friday, Saturday and Monday.