This means tickets for the 24-horse race can cost no more than $20.83, according to a spokesman.
Any money raised must be returned as prizes and no one is allowed to profit from organising the sweepstake.
Violating these regulations could incur a fine of up to $1000.
And beware of offering prohibited prizes. Firearms, liquor, tobacco and vouchers for sex services are all out of bounds.
Those lucky enough to skive off work early will be packed into bars or attending countless cup celebrations at racing clubs and Returned and Services Association rooms.
Kiwi punters are expected to bet about $8 million on today's big race. One of the largest bets placed yesterday was for $5000 on Australian horse Ethiopia, a TAB spokesman said.
"If it wins, the lucky punter will stand to get something in the vicinity of $100,000."
In 2011, 14 per cent of Kiwis aged over 18 placed a TAB bet on the cup.
This year, four New Zealand-bred horses will line up at 5pm (NZT) to compete for the $7.77 million prize money.
Maluckyday, Lights of Heaven, Precedence and Zabeelionaire are among the 24 stayers entered in today's race. Patriotic punters will be closely watching Maluckyday, who came second in the 2010 cup.
New Zealand champion jockey James McDonald is down to ride Irish horse Fiorente.
Yesterday, reigning champion Dunaden was the race favourite. Commentators also expect French-trained Americain, who took out the cup two years ago, to put up a tough fight.
Office sweepstake rules:
• Prize must not exceed $500
• Any money collected must be returned through the prize
• Prohibited prizes include:
• Firearms
• Liquor
• Vouchers for sexual services
• Tobacco products
Source: Department of Internal Affairs
Winning tips:
• The race has been won most often (44 times) by a horse aged 4 years
• The most successful sex is "entire", aka a male horse which has not been desexed (64 wins)
• Numbers 4 and 12 are the luckiest numbers (11 wins each)
• The race has been won six times each by horses starting from barriers 5, 10, 11 and 14
• Jockeys wearing predominantly black or royal blue have been the most successful
• The race has never been won by a horse starting from barrier 18