Forum media co-ordinator Derek Fox said Mr Tetava's rule meant all security staff travelling with leaders would be told to hand their guns over on arrival in the Cooks.
He said all security would be in the control of the Cook Islands police, and only those sworn in as Cooks officers could carry firearms. Some police from Australia and New Zealand would assist.
"I understand the Commission of Police from the Cook Islands has already advised other jurisdictions that should they arrive with firearms, the Cook Islands police will take hold of them and put them in a safe place until they go home and then they will be given back," said Mr Fox.
He did not know whether the United States was concerned about the rule, but said the Cook Islands was an independent country.
Security personnel from Australia, New Zealand and the United States were already in the Cook Islands checking arrangements in advance of the leaders' arrival on Wednesday.
New Zealand is sending over 10 police officers to assist. A spokesman said they would serve in a range of roles, including search specialists to check venues in advance, a dog handler and a radio technician. None would carry firearms.
The firearms issue is the latest in a series of logistical considerations prompted by Mrs Clinton's proposed visit. Organisers told the US to send a smaller delegation than the usual 90 as accommodation on Rarotonga was already booked out. A private home is understood to have been booked for the stay.
The Associated Press reported that organisers had had to borrow SUVs from island residents for motorcades, and the Cook Islands News reported that while there was just enough room for all the VIP planes at the airport, if the cargo plane that sometimes accompanied Mrs Clinton's VIP plane was also coming, it might have to stop in Pago Pago, in American Samoa, about two hours away.
Mr Key will be travelling in a Royal New Zealand Air Force Boeing 757, Ms Gillard in a Royal Australian Air Force VIP Boeing 737, and there will be smaller planes carrying Papua New Guinea Prime Minister Peter O'Neill and New Caledonian President Harold Martin.
Mr Fox said speculation that the United States was sending an aircraft carrier or other warships closer to Rarotonga had not been substantiated.
The United States is one of 14 "dialogue partners" who regularly send officials or politicians to the annual Pacific Islands Forum, where talks with them are held on the last day.