Captain Percy Morokane said the owners of the home were digging on the property in order to install a swimming pool when the discovery was made on Tuesday evening.
They immediately contacted the police.
Forensic experts visited the site and the material would be sent for forensic testing, said Morokane.
"What we would like to say is to let the investigation go on, we want the police's work to go unhindered. We know the country at large is waiting in anticipation - we ask for patience," he said.
Van Rooyen and his accomplice and partner, Joey Haarhoff, were linked to the disappearance of six girls between 1988 and 1990.
They allegedly kidnapped Joan Horn, 13, Odette Boucher, 11, Anne-Marie Wapenaar, 12, Yolande Wessels, 12, Fiona Harvey, 12 and Tracy-Lee Scott-Crossley, 14.
Van Rooyen killed Haarhoff and then committed suicide in January 1990.
The knowledge of the whereabouts of the missing girls died with the couple.