Land co-owned by accused murderer Ron Medich has served as an illegal dumping ground for 65,000 tonnes of waste, some contaminated with asbestos.

Tenants of the once-wealthy property developer and his brother Roy added repeatedly to the dump at Badgerys Creek in Sydney's west since it was discovered in 2015, the Environmental Protection Authority found.

Deadlines to introduce safety precautions and clear the site have been missed. When Fairfax Media accepted an invitation to visit on Sunday, a large mound bore a sign saying "Asbestos Keep Away" but the protective plastic sheeting had blown off.

Estimated to be worth hundreds of millions of dollars if rezoned, the land has split the brothers in an ongoing court case about the right to sell it. The 344-hectare block has also previously featured in corruption inquiries and as part of an alleged blackmail attempt on Mr Medich by the murdered businessman Michael McGurk.