As Protectionist romped away to a four-length victory in last year's Melbourne Cup, Greg Carpenter was already thinking about his weight as a defending champion.

The handicapper begins constructing the weights for the next Melbourne Cup as soon as the field goes past the winning post.

"Quite honestly when Protectionist raced away and won by four lengths, I was thinking about what weight he was going to get in the 2015 renewal," Carpenter says.

That's the starting point for the living document that is the Melbourne Cup weights, continually updated after races around Australia and overseas.

Already a complex task, constructing the weights for this year's Melbourne Cup threw up last-minute challenges.

"If you work for 12 months on the Melbourne Cup weights and then all of a sudden within the space of 24 hours there's some dramatic performances that change the whole structure of the weights, you're working right up until the death knell," Carpenter says.

Racing Victoria's chief handicapper says the all-important weights went right down to the wire after Irish trainer Aidan O'Brien's Order of St George staged an impressive 11-length victory in the Irish St Leger that followed up Bondi Beach's win in the English St Leger.

Then there are the entries vying for a British stable's first Melbourne Cup victory including Goodwood Cup winner Big Orange and the narrowly-beaten third-placed Trip To Paris, the Ascot Gold Cup winner and stablemate of Red Cadeaux.

"They're a difficult group of horses to assess when they are sharing victories and exchanging orders of finish," Carpenter says.

Carpenter agonised over three-time Cup runner-up Red Cadeaux, UK trainer Ed Dunlop's popular British stayer that is pursuing his fifth Cup start in a row.

"He's a 10-year-old - no horse older than eight has ever won the Cup before. He's been here four times and hasn't been able to win it.

"So it was a really vexed question and something I spent a long time thinking about what weight he would get."

Carpenter is only the ninth person to have the job of deciding the weights for the Melbourne Cup, a task he describes as a privilege after releasing his 11th list.

A lot rests on the handicapper's decisions, particularly for the topweights for the $6.2 million Cup.

In 22 attempts, no original topweight has placed in the world's richest handicap since Comic Court won the 1950 Cup.

Carpenter, for one, would love it if defending champion Protectionist or English star Snow Sky could break that 55-year hoodoo.

"The handicapper always likes to see the highest weighted horses in the finish so I'm hoping that Protectionist and Snow Sky will be able to deliver that for me this year."

He admits the handicap presents a challenge to both Protectionist and Snow Sky, but believes they have earned their position at the heads of the weights based on their performances over the past year.

With the weights released seven weeks before the first Tuesday in November, there are still challengers vying to qualify to get into the field.

"The puzzle is out there now for everyone to solve. What they have to do is track their way through to the Melbourne Cup," Carpenter says.

"A lot plays out between the day the weights are released and the first Tuesday."