Organisers of a charity event for elderly citizens have pleaded for the return of more than $20,000 in cash stolen from a function at Crown Casino.

The cash was stolen from behind a table in the Palladium Ballroom, about 11pm (AEDT) on Sunday, during a charity auction staged by the Elderly Citizens Help Organisation (ECHO).

Police say about 60 royal blue envelopes containing the money went missing during the event, after they were collected for a Melbourne Cup sweep.

Detective Sergeant Barry Jenks, from Melbourne CIU, said the offender was an "opportunist" who most likely wandered into the ballroom.

"The offender involved may or may not know the funds he was taking in fact belonged to a charity," he said. "So we'd be very grateful, obviously, if the money was returned to the charity fund.

"This organisation, they're all public spirited people who help out the elderly in the community and they're all very, very distraught that the funds have been stolen."

Detective Sergeant Jenks said police were working with the casino to investigate the offender's possible movements.

"There's an exit and entry point pretty close to where the table was, so we're working on the theory that most of the people inside the function were engaged in the auction," he said. "It's likely an opportunist came past and struck gold."

ECHO president Ian McGregor urged whoever was responsible to return the money, which would be used to support elderly citizens across Melbourne.

"I was sick to think somebody would steal from a charity when during the night I emphasised exactly what the money was raised for," he said.

"Strangely enough, it was a record attendance, a record calcutta pool and people totally supported us during that night."

Mr McGregor said the event was a major fundraiser for ECHO and the theft would have a big impact on the amount of money the organisation could spend over the next year.

"I just couldn't understand why someone would want to do that to a charity," Mr McGregor said.

"One day they might be elderly citizens themselves and might be in need."

Security would be increased at future fundraisers, he said.

Mr McGregor said ECHO, which formed in the early 1970s, raises up to $100,000 annually for spending it on the elderly on equipment such as wheelchairs and beds.

He said the organisation would still be responsible to pay out the winnings to those who took part in the auction.

Police called for anyone with information to make contact.