With players sweltering in 36 Celsius (96.8 Fahrenheit) temperatures, the tournament's five-part heat stress scale hit five.
The scale measures four climate factors: radiant heat, air temperature in the shade, relative humidity and wind speed.
The lowest level reading is one and the highest five, at which point play is suspended and the roofs are closed on the three main stadiums - Rod Laver Arena, Margaret Court Arena and John Cain Arena.
The heat led to a three-hour interruption in the afternoon of round one matches on outside courts.
Conditions improved and play was able to resume, but then a storm hit, dumping torrential rain that forced players back into the locker room.
About 90 more minutes were lost and while some matches resumed, at least eight that were yet to start were cancelled and will now be played on Wednesday, organisers said.
They included those featuring women's eighth seed Daria Kasatkina, 14th seed Beatriz Haddad Maia, and American Sloane Stephens.
