Extreme hot weather in Perth sent the internet into meltdown on Monday night.

Thousands of iiNet customers across Australia found themselves offline for about six-and-a-half hours after the company shut down some of its systems at its Perth data centre at about 4.30pm AEDST because of high temperatures, topping 44.4C.

"Due to record breaking temperatures, iiNet Toolbox, Email and our corporate websites are unavailable. Apologies for any inconvenience caused," iiNet tweeted.

Customers trying to re-authenticate after disconnecting from the internet were also unable to reconnect.