Horses were being led to the barriers. Televisions were being switched on everywhere.

And in Darwin, far from the rails at Flemington Racecourse, a data room full of powerful computers was processing a truly staggering volume of bets from punters far and wide.

Right before the Melbourne Cup, between 2.55pm and 3pm, "that's when the beads of sweat start forming," says Simon Noonan, chief information officer at the nation's biggest online bookmaker, Sportsbet.

"There is no other business like this in Australia, having this one event where the time is so critical and demand is so simultaneous."

The Melbourne Cup is the biggest betting event on the Australian calendar, and the sheer number of bets placed in the lead-up is extraordinary.

New wagering data reveals a new record was reached in the immediate lead-up to this year's race, with 850 bets placed through Sportsbet during the busiest single second (an increase of 70 per cent from the previous year). The busiest single minute saw 26,000 bets placed (30 per cent higher).

The foreign-owned Sportsbet, like all Australia's major internet bookmakers, is based in the Northern Territory, where there is a lower-tax regime and more favourable regulations. All online bets placed through Sportsbet are received through the company's data room in Darwin.

Overall transactions, including bets, deposits, withdrawals and returns, peaked at 17,000 per second. For the day, there were 360 million Sportsbet transactions.

"In that one 30-minute period, things go off the Richter scale," Mr Noonan says. "But our systems didn't creak ... the platforms we've created allowed us to have the confidence that 'we've got this'."

The website and app of another major internet bookmaker, Ladbrokes, crashed during the lead-up to the Melbourne Cup, setting off social media posts from punters.

"We deeply apologise for the inconvenience and understand the frustration," Ladbrokes replied on Twitter.

Punters with TAB placed nearly 11 million bets on Cup Day and wagered more than $90 million on the Melbourne Cup itself.

Tabcorp said more than 914,000 people visited Tabcorp's app and site during the day, with a peak of more than 185,000 digital users at any one time.

Sportsbet, whose digital wagering turnover has outstripped that of TAB, has recently expanded its technology team to about 300 staff.

Mr Noonan said Sportsbet used Amazon Web Services and IBM to "scale up" its infrastructure capacity in order to meet the huge spikes in demand, especially for the massive Melbourne Cup.

"We have now got the people in the place and the techniques in place that we can scale up," he said.

"Some of the infrastructure we put in place we increased 1500 per cent on Cup Day."