Wrexham Council has also issued a temporary stop notice after a “significant amount” of material was deposited “without planning permission”.
The council said it would also be serving enforcement and stop notices “to remedy the situation and have the land restored”.
Tom Lawson, from Coedpoeth, said he was frustrated that the activity was not stopped by NRW when it was first spotted.
He estimated there were thousands of tonnes of material, which he said looked like “low-grade demolition waste”, saying that he had seen tar and lead batteries in amongst the rubble.
He said he also feared the people responsible would “just walk away from it”.
“It’ll be left for the public to pay… it’s just wrong,” he said.
Wrexham-based company SOS Aggregates said it owned the land in Coedpoeth and had been depositing “grade 1A fill material” on the site to make a plateau on which to create the equine arena which would then be “landscaped”.
A spokesman for the firm denied it had continued depositing on the site when the temporary stop order was in place, but said it had continued work once the order expired.
He said that they had told the council they would be submitting a planning application this week and that they were also seeking legal advice.
He said the company would also be providing evidence to NRW of the materials being used on the site.
The investigations in Coedpoeth come as another field on the edge of the nearby village of Minera has seen dozens of lorries depositing materials, which one local councillor described as “reeking of diesel”.
Caravans have also been placed on the site opposite a cul-de-sac of bungalows on Nantlais.
The farmland was sold approximately two months ago, but according to the council no planning application has been submitted for development there.
SOS Aggregates said it had delivered recycled construction material to the site in Minera, but that it was not involved in work taking place there and did not own the land.
Wrexham Council has served a temporary stop notice due to “a breach of planning control and the change of use of the land to a gypsy and traveller site with associated engineering operations”.
NRW said it was investigating the materials on the site and would take action if appropriate.