THERE are fears the health of a former Hong Kong couple kidnapped by guerillas in Cambodia may be rapidly deteriorating. The Sunday Morning Post learned yesterday that Briton Dominic Chappell, 25, suffers from asthma and Australian Kelly Wilkinson, 24, requires regular treatment for water retention. Emergency medical supplies have been sent to them through intermediaries but it cannot be confirmed the couple have received them. However, while he did not comment on their medical conditions, Mr Chappell's father, David, maintained the couple were tough and were being well cared for by their captors, who were almost certainly members of the Khmer Rouge. ''Dom and Kelly will be OK, they know the Khmer people and speak Khmer quite well,'' Mr Chappell said. The pair were abducted three weeks ago as they drove down the now notorious Route Four to the southern port of Sihanoukville where they run a restaurant. Also abducted was an English tourist, Tina Dominy. According to locals it is likely they are being moved regularly for security reasons. Miss Wilkinson was not wearing shoes when she was taken. It is understood the Cambodian Ministry of the Interior and the army, which are organising the search for the three, are using established networks of undercover police and soldiers. But no official contact has been made yet and yesterday David Chappell expressed concern over the way the rescue operation was being conducted. Speaking in Sihanoukville before returning to Phnom Penh he said there appeared to be no ''central co-ordination'' between the police and the military. He said he was happy with the way the police were conducting the operation, but had concerns about the overall plan. Officials from the Australian and British embassies in Cambodia have sent staff to Sihanoukville to act as advisers to the Cambodian authorities. Two Australian federal police officers who speak Khmer have also been sent from Australia to assist. The pair moved to Cambodia in July from Hong Kong. David Chappell, a writer-photographer from Lamma Island, arrived last week for a special meeting with King Norodom Sihanouk. The monarch assured him that his son was safe and appealed for the Khmer Rouge to release the trio.