A Daley Labor government would review the emissions standards of all NSW's coal-fired power plants and finalise a 10-year plan to improve air quality in the state.

The pledge came as Sydney weathered its hottest day in a year on Thursday, with temperatures topping 39 degrees at Observatory Hill and 40 degrees in the west, capping what was most likely the city's hottest January ever.

In the run-up to the expected jump in electricity demand, a generation unit at the AGL Liddell plant and one at Origin's Eraring plant failed, driving up wholesale prices to more than $14,000 an hour, underscoring the fragility of the sector.

EnergyAusralia's Mount Piper Power Station near Lithgow: no change to its pollution licence. Credit:Nick Moir

"As our fleet of coal-fired power stations comes to the end of their lives, they will become increasingly unreliable, especially in extreme weather situations, which are becoming more frequent due to climate change," Adam Searle, Labor's energy spokesman said.