"I left a message on his phone, he didn't respond, so I emailed him.
"Sadly he is immune to reason and considers the potential enjoyment of the 40 children and more than 80 adults to be of greater importance than serious trauma and potential catastrophic loss of life," Frewin said.
The display was planned for this Saturday night.
Grant said the fireworks would be going ahead and he had every right to do it.
"I sent a warning notice to all my neighbours within about 4km, giving them plenty of time to move their horses if they were concerned. I've done everything within my legal right."
But Frewin said they couldn't move the horses because they would need a stock cart and they are difficult to get at this time of the year.
"It does raise a bigger issue about commercial-grade fireworks being used for private use and it does need addressing," Frewin said.
Grant said circulating his email and phone number on Facebook was out of line.
"If you react in this way and act like this, then people won't be so considerate to give notice."
The debate took to social media last night triggering bursts of mixed opinions.
Some said Grant had done everything right; others said it was inconsiderate.
Frewin, who posted the issue on the Kaimanawa Heritage Horses facebook site, took the post down this morning, saying it was not her intention for people abuse or threaten Grant.
"His party is permitted and he's not doing anything illegal. My only intention was to demonstrate just how many people nationwide are concerned about the welfare of NZ's wild horses," Frewin wrote.
I have deleted the post about the fireworks party in Waitoki.
It was never my intention to have people abuse or...
Posted by Kaimanawa Heritage Horses on Wednesday, 6 June 2018
In the post, she explained the horses were already traumatised after being "ripped apart from the families" and transported long distances.
"These aren't just a few sparklers for 5yo's, these are commercial display grade fireworks," she wrote.
