Blood tests taken from a horse that died suddenly at Randwick Racecourse have returned negative results to the Hendra virus.

Monday's death of the Anthony Cummings-trained Rivo Boy, which was being treated for an illness, prompted Racing NSW stewards to enact bio-security measures.

The restrictions were relaxed in the afternoon when the blood results were known.

Stewards said Rivo Boy will now be the subject of a necropsy report to be performed by the University Teaching Hospital at Camden to determine his cause of death.