MetService meteorologist Tom Adams said the nearby Hunua Ranges registered 180ml of rainfall in the 24 hours to noon yesterday.
Ms Ross said her family woke on Wednesday to find a "river flowing through one of our big sheds", with other sheds damaged by water flowing from the Wairoa River - which runs next to their property.
Ms Ross said it was "very disappointing" to miss this year's event but "we've got a lot of work to do sorting out the insurance claims".
The champion equestrian said she also had to consider the health of her horse - Parkridge Disco SW, known as Danny.
There had been a lot of time and effort put into training the 9-year-old gelding, and she did not want to risk him being injured on the "sodden" grounds at HOY.
"It's very frustrating because you work really hard to get to these big shows ... it's a real shame to miss it but at the end of the day we have to think of the horse," she said.
"You don't put your horse at risk for one show, even if it's one of the best shows in the country."
Weather was also thought to be a factor in a crash that closed State Highway 5 between Napier and Taupo for nearly three hours yesterday morning.
Police said a truck "jack-knifed" and blocked the road at Tarawera, just after 6am.
There were no injuries, but the road was closed until about 10.30am, leaving some motorists trapped in Taupo.
One of these was Jody Hartstone, a top-flight dressage competitor who has competed in HOY several times.
She said they had waited in Taupo for about half an hour before the road was reopened, arriving in Hastings later than expected.
The cause of the incident was unknown at this stage.
HOY event director Dave Mee said he had not heard about competitors not being able to attend, or being delayed because of the weather.
However he said usually 15-20 per cent of competitors were unable to attend the event, or made last-minute changes "for a whole range of reasons".
The skies over HOY were not expected to clear today. The forecast today was cloudy with a few showers, turning to rain with possible heavy falls, and a high of 20C.
Periods of rain were expected to continue on Friday, with possible heavy falls about the ranges. Northeasterlies were forecast, becoming strong in exposed places.