An Australian mother has said she believes her family's pet dogs kept her four-year-old Down syndrome son safe after he went missing overnight.

Cold, lying face down and covered in scratches, Riley Martin was found sitting on the ground in undergrowth in the state of New South Wales after he disappeared for almost 24 hours.

He and his three pet dogs - two fox terriers and a kelpie - had not been seen since midday on Wednesday when he went missing from his home in the rural town of Nambucca Heads.