Australia's most famous horse race, the Melbourne Cup, is the race that stops two nations.

And while workplaces may halt to a standstill at 5pm tonight, Internal Affairs is warning bosses on how to keep their sweepstakes by the book.

Office sweepstake prize money cannot exceed $500, which means tickets for can cost not more than $20.88.

And for anyone with an entrepreneurial streak, any money raised must be returned as prizes and no one is allowed to profit from organising the sweepstake.

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Violating these regulations could incur a fine of up to $1000.

And beware of offering banned prizes. Firearms, liquor, tobacco and vouchers for sex services are all prohibited.

"If you are thinking of running a sweepstake you can – the hurdles are not too hard and make a lot of sense— but do make sure you understand and comply with the rules," the DIA warned yesterday.