Twenty-one started the two-round competition over the course built by German designer Werner Deeg. The going was tough and certainly took its toll on the horses and riders, with rails falling around the course.
The crowd went wild when Tootill and Ulysses NZPH delivered the only clear of the first round.
Those on eight faults or less - nine combinations - came back for the second round over a shortened course.
There were five clears in that second round, coming from Samantha Morrison (Tauranga) and Biarritz and Katie Laurie (Mystery Creek) with Dunstan On the Point Eve, both carrying eight faults, and Maurice Beatson (Dannevirke) aboard Mandalay Cove and defending champion Helen McNaught-McFarlane (Taupo) with Carnutelabryere, who both brought four faults through.
McNaught-McFarlane stopped the clock in 58.6 seconds and Beatson was also clear but slower, in 61.19.
Tootill came out knowing she had to jump clear, but if not, she needed to be faster than McNaught-McFarlane.
"The time was quite tight, which helped me keep things flowing," said Tootill.
But she needn't have worried, as in the end all the rails stayed up, with McNaught-McFarlane and Beatson second and third respectively. Laurie was fourth, Morrison fifth and the youngest competitor, 17-year-old Emily Hayward-Morgan (Te Awamutu) aboard AP Ninja sixth.
Tootill had a great show, also coming second in the Young Rider of the Year and sixth in the Lady Rider class, but it was the Olympic Cup that really topped it all off.
The combination is also part of the New Zealand team to compete in Australia later this month, and today's win gives her plenty of confidence heading into the tour.
Taranaki's Abbie Deken (Taranaki) had the biggest win of her career, riding off with the Dressage Horse of the Year crown on her lovely chestnut KH Ambrose.
It hasn't been an easy road for the pair, but all the hard work paid off for Deken, who did a personal best in today's musical freestyle to clinch the title.
KH Ambrose is a 14-year-old gelding by Anamour and was bred by John McGrath. Deken bought him as an unbroken three-year-old.
Deken's goal for the show was to improve on her nationals performance and ride better. She and coach Vanessa Way had worked hard on the passage piaffe work, and that paid off today.
"I had a nice feeling throughout the test and a smile on my face," said Deken. "I went into it thinking, 'let's try to enjoy it', and I did. To get the win on top of that is the icing on the cake."
It was a battle between Deken and John Thompson (Gordonton), who won the grand prix special yesterday and looked on track to defend his title.
But Deken and KH Ambrose had a blinder in the CDI*** FEI Grand Prix Freestyle today, scoring 72.9% for the win and sealing the overall Dressage Horse of the Year title for 2017.
Thompson and JHT Antonello were second in the musical freestyle today with 71.825% and second overall. Third overall was Penny Castle (Ashhurst) with Magnus Spero and fourth was Jody Hartstone (Raglan) and Ali Baba.
HOY director Dave Mee said the weather was against organisers but overall felt the event went well.
"When the weather is against you it's really challenging to be on your toes and find solutions," Mr Mee said.
Feedback from the eventing riders that they didn't feel it was safe to jump meant it was quite a simple decision to cancel the eventing yesterday in the end, although they did explore every possible option to keep the event alive.
Mr Mee said the two events today, the Olympic Cup and Dressage Horse of the Year, were classes they needed to protect and only three classes were cancelled.
"I think that was a good outcome. In more than 20 years in events this would be the wettest I have ever seen."
Going into the event pre-sales were up 30%, which was a good sign. Unsurprisingly, Saturday gate numbers were down on last year.
"I think we will come out similar to last year (numbers wise). We are quite relieved, to be honest."
Although the show had insurance policies, unfortunately the insurance would not kick in, as the rain fell at the wrong time.
"Despite the weather, coming in to our second time organising the event we still had a long way to go, but we felt more confident in the equestrian space.
"We've had positive feedback from sponsors and also the non-equestrian events worked well - we want the event to be something the whole community can get behind."
In other showjumping classes, Christchurch's Steffi Whittaker and Moonlight Glow were the only double clear in the two-round Pony of the Year class, lifting the Somerset Fair Cup.
An emotional Whittaker was clearly thrilled to win the class, thanking her granddad and nana for giving her the pony in front of a packed grandstand.
"I told granddad before we left, if I don't get it this time I'm never going to get it," said Whittaker.
Local rider Ruby Mason and Mr Acho was the only four-faulter to jump a clear second round, adding nothing to her first round tally, to finish second.
Waiuku's Tyla Hackett and Gin emerged victorious in a very competitive Junior Rider of the Year class, blitzing the jump-off with a clear round and stopping the clock in 26.85 seconds.
In the dressage, Caitlin Benzie (Pukekohe) and Rosari Royal Gem were the Young Rider of the Year for 2017.
Vanessa Way and NRM Andreas scored 67.7% to win the Harney & Sons Musical Freestyle and clinch the overall Advanced Dressage Horse of the Year title.
In the Level 8 Dressage Horse of the Year competition Susan Tomlin (Palmerston North) and Dancealong were the winners, winning all three tests.