Persistent rainfall that started before the event kicked off at 11am resulted in the Livamol Classic being abandoned just before 4pm after safety concerns for horses and jockeys were raised.
Mr Castles said although the weather didn't put off racegoers, it affected the races to "the ultimate extent" after officials were forced to abandon the meeting after five races.
"Unfortunately that's the thing with outdoor sports and events; you are exposed to the vagaries of the weather and Mother Nature conspired to get us on Saturday."
Despite the dreary weather and afternoon call-off, Mr Castles said the event continued and many stayed well past 4pm as bars, catering options and betting on other races outside of Hawke's Bay continued.
"Even with the bad weather the crowd were in great spirits. There was a wonderful crowd and I think it was a great event but unfortunately the weather gods conspired against us."
The chief executive said he was yet to have an event debrief with police, but understood the crowd was fairly well-behaved.
"From my understanding and talking to my staff, they were of the belief that the crowd was pretty well-behaved across the board.
"At an event of that size and nature you're always going to get the odd issue but we had a really strong security presence through our security partner, Red Badge."
The result meant Hawke's Bay Racing was set to be dealt a financial blow in the tens of thousands, he said.
"It's going to depend a little bit on whether we can relocate the race. If we're able to relocate the Livamol then we'll be able to mitigate some of the loss but at this stage I'm not really in a position to quantify that."
