The Korea Institute of Nuclear Safety (KINS) said no unusual levels of radiation had been found as of early Sunday.
"We have detected no unusual level of radiation from air samples. We are waiting for the results from analysing East Sea (Sea of Japan) water samples to come out," KINS spokesman Kim Sang-Hyun told AFP.
He said that was expected by Tuesday.
North Korea declared on October 9 it had carried out a nuclear test, with seismic readings confirming an earthquake-strength blast from the communist state's remote north-eastern area.
South Korea, along with the United States and Japan, have since been trying to verify the announcement independently.
Unnamed US officials said on Friday that initial findings showed unusual levels of radiation in air samples taken by a military drone on October 10, suggesting an atomic device.
But Japan on Saturday reported no abnormal radiation on its territory.

Copyright AFP (Agence France-Presse), 2006