Melbourne will be lashed with up to six hours of heavy rain on Thursday night, but the weather bureau says the downpour is unlikely to delay play on day five of the Australian Open.

A severe weather warning has been issued for half of Victoria, with rain and flash flooding forecast from Melbourne to the South Australian border.

Senior forecaster Richard Carlyon said up to six hours of heavy rain was forecast for Melbourne overnight.

"The issue for Melbourne is heavy rain [on Friday] morning, likely to commence around 4am and finish around 9am," he said.