Rain and flash-flooding have hit Flemington just hours before the Melbourne Cup race as punters brace for a deluge.

Bureau of Meteorology senior forecaster Michael Efron said 17.4mm of rain fell in Melbourne in the four hours up to 11am on Tuesday, and up to 10mm was still on the way.

The heavy rain has also seen the Flemington track downgraded to a heavy eight, with races two and three pushed back.

That’s good news for late arrivals who might find it difficult to get to the racecourse due the weather playing havoc with Melbourne’s public transport system.

Flooding near Flemington is causing train delays of up to 20 minutes and a number of inner-city tram services have been stopped because of storm damage.

Interstate visitors planning to jet out later in the day might also face delays, with Air Services Australia tweeting that Tuesday morning’s poor weather could cause problems.

“We can expect showers during the race but it’s unlikely it will be as heavy as this morning,” Mr Efron said.

“We will start to see showers easing in the afternoon but are still likely to see showers during the main race as well.”

It might also be a good idea to pack a warm coat, with southerly winds forecast to bring cold temperatures in the late afternoon, as punters begin heading home.

“People heading home from Flemington are likely to have a pretty cold trip,” he said.

It could be particularly cold for flamboyant British billionaire Marwan Koukash who has vowed to accept the trophy in his G-string if his horse Magic Circle wins the race.

The horse is soaring towards being the race favourite, now sitting at $6.

Earlier favourite Yucatan has dropped from $5.50 to $8.

“That’s the lace that’s going to stop the nation,” Koukash told AAP at Monday’s Cup Parade.

A severe weather warning is in place for Melbourne and eastern parts of the state, as a slow moving low pressure trough brings heavy rain and thunderstorms.

Flooding, storm leads to Vic power outages

Heavy rain has caused flooding, building damage and fallen trees across parts of Victoria where the State Emergency Service is responding to hundreds of calls for help and thousands of properties are without power.

More than 370 calls have been made to the SES in the 24 hours to 12.30pm on Tuesday with 283 requests still outstanding. The majority are for flooding and building damage.

About 4300 Victorians are without power due to lightning strikes, the energy companies’ websites reveal.

Wild thunderstorms causing flight chaos

Thunderstorms and heavy rain have caused flight chaos to and from Melbourne.

Passengers arriving on a flight from Singapore along with those on a Qantas service from Japan had to be diverted to Sydney on Tuesday after heavy rain and cloud forced a runway closure, Melbourne Airport spokesman Grant Smith said.

“Poor weather this morning @Melair may cause some delays this afternoon. Weather is expect to improve later in the day. Passengers are advised to check with their airline for details,” Air Services Australia tweeted.

A Virgin Australia spokeswoman said the Melbourne Airport tarmac was closed for 45 minutes on Tuesday morning, leading to flight delays of up to an hour across most airlines.

“We are getting out as many flights as possible. We are trying to keep the effect on (Melbourne Cup) racegoers to a minimum,” she said, noting there had been no cancellations but a few minor delays.

Five of Jetstar’s domestic return flights were delayed for more than an hour but the company is now running on schedule, a spokesman told AAP.

Almost 35mm of rain had fallen at Melbourne Airport by midday, with the weather bureau issuing a severe weather warning for Melbourne and eastern parts of the state.