Players were to complete their current sets before a suspension until at least 5:00 pm (0600 GMT) on outside courts, they said, while the roofs were closed on the centre and second courts.
"The Australian Open extreme heat policy has been implemented," said a statement.
"The roof on Rod Laver Arena and Hisense Arena will be closed at the end of the set. Play will be suspended at the end of the set on all outside courts until further notice."
Temperatures were forecast to reach 44 Celsius on Thursday, the hottest this week, and remain high on Friday before cooling sharply on Saturday.
The tournament's opening days have been dominated by problems associated with the heat, with players falling dizzy, fainting and vomiting, and warnings of serious health problems.
"Doing physical exercise in this heat is just unbelievable," said France's Alize Cornet. "Even for the fans, sitting in the sun must be terrible."
America's Varvara Lepchenko was the latest victim on Thursday when she had her pulse and blood-pressure checked and was rubbed down with ice by staff during a medical time-out.
Temperatures had already reached 38 Celsius as play started at 11:00 am, with players shrouding themselves in ice packs and gulping drinks at the changeovers.
