Green Moon has pulled up lame from his failed attempt at back-to-back Melbourne Cup wins.

Last year's champion was striving to become just the sixth horse to win multiple Melbourne Cups but came in 21st, more than 40 lengths from the winner Fiorente.

His jockey Brett Prebble immediately feared the horse had a problem.

"He went amiss and pulled up quite distressed," Prebble said.

"Hopefully all is OK but he was definitely distressed, the poor bugger."

Stewards reported Green Moon was lame in his left front leg after the Cup.

Prebble flew in from Hong Kong last year to win the Cup on Green Moon at his first ride on the Lloyd Williams-owned import.

He has ridden him in every start since and went into Tuesday's race thinking Green Moon was a winning chance.

Green Moon was one of six Cup runners for Williams with Caulfield Cup winner Fawkner doing best of the sextet for sixth.

"It was a great effort from all our team (to have six runners). Unfortunately they didn't run up to expectations, but that's life," part-owner Nick Williams said.

"Next year we'll come back stronger and faster."

Williams said the day was "all about Gai" after champion trainer Gai Waterhouse trained her first Melbourne Cup winner.

"She hasn't had a great year and I just think it's the best thing for Australian racing but more importantly, it's the best thing for her," Williams said.

"And I don't think there's anyone that deserves it more."