Thousands of Victorians were left power after a slow-moving storm caused flash flooding in Melbourne, with half of the city's November rainfall arriving in just three hours.

More than 30mm of rain fell after 9am on Melbourne Cup Day, causing train delays and soaking racegoers at Flemington.

Flooding has caused chaos along the Kingsway, at the tunnel entrance onto the CityLink, as well as in parts of Southbank, Rowville, Balwyn North, Windsor, Hawthorn, Ashburton and Narre Warren.

Cars were left submerged, with some motorists forced to push their cars through the floodwaters.

The SES received more than 400 calls for help in the past 24 hours, mostly for flood related emergencies.

Olympic Park received 35mm of rain before 12pm, while Melbourne Airport copped 34.6mm.

"It's a very slow moving system," Weatherzone meteorologist Joel Pippard told 9news.com.au.

"It's a big long line of heavy rain."

Flash flooding also caused chaos on the train lines with service to Flemington Racecourse temporarily halted.

Crews were urgently sent in to clear the water before the race finished this afternoon.

This morning, flights were diverted from Melbourne Airport because of unsafe landing conditions.

A Jetstar flight from Sydney flew all the way to Melbourne before returning when it couldn't land.

Meanwhile, three selfless strangers have raced to the aid of a bogged driver in the city's south-east.

Video shared to 9News shows the trio using their four-wheel-drive to pull the small silver Ford car out of flooded Sweeney Drive in Narre Warren.

Melbourne today received more rain in three hours than in all September and October together.

Weatherzone has confirmed this is the second wettest Melbourne Cup day on record.

The previous highest amount of rain recorded during the 24 hours from 9am on Cup Day was 46.5mm in 1942, when the race was run on a Saturday.