Perth has sweltered through its hottest night in two years.

After the mercury reached 40 degrees yesterday, there was little relief overnight with temperatures remaining in the mid to high twenties.

At 5:00 am today the mercury was at 29.2 degrees, before the city's minimum of 25.2 degrees was recorded just after 6:30 this morning.

The city was treated to spectacular light show as thunderstorms rolled across the Metropolitan area last night, but there was little rain recorded.

Perth's temperatures are tipped to reach 38 degrees today.

Meanwhile, a lightning strike has affected the Joondalup train line, affecting hundreds of peak period passengers.