Natural Resources Wales (NRW) is asking farmers and land owners to be aware of requests to store waste on their land.
It follows recent incidents where farmers across Wales have accepted waste for temporary storage.
This is a growing problem driven by criminal gangs.
The type of waste can vary a lot, from partially shredded plastics and materials, to mattresses and even soil or building material.
Landowners are reminded that storing waste is tightly regulated.
And any waste exemptions have strict conditions about the amount and type of waste that can be stored.
The rules are in place to make sure waste is managed and stored correctly to avoid pollution and fire risk.
Euros Jones, North West Wales Operations Manager for NRW said: “If individuals or companies offer baled waste to farmers to store, supposedly on a temporary basis, it is vital they check that the right permissions are in place. Waste exemption rules can be checked on our website.
“Landowners may let land in good faith to waste operators, but it is vital they do so with their eyes wide open.
“There are unscrupulous operators looking to dump waste anywhere they can. The land owner is often left with significant quantities of waste, which is potentially damaging to the environment, and a large bill to remove it legally.
“Anyone approached to store baled or any other type of waste on their land should report it to us immediately on 03000 65 3000.”
NRW is continuing with investigations into several sites across Wales.