Two Chinese tourists have been arrested for allegedly trying to post parcels of heroin from Thailand to Australia.

Ka Sing Chan, 28, and Chor Lung Lui, 19, from Hong Kong, reportedly posed as clothing vendors mailing 18 boxes of shirts to buyers in Australia on October 9.

However, staff at the shipping company in Bangkok's Chatuchak Plaza became suspicious as the pair could not keep their cool and appeared to be 'unusually hasty' to have the parcels sent. They called the police and slowed down the service until the cops could arrive.

Police Colonel Phuvadol Unpho, superintendent of the Bang Sue District police said officers opened up the packages and found a total of one kilogramme of heroin inside.

The contraband had been hidden at the bottoms of the boxes and concealed with black sheets with the shirts placed on top.

Police seized the illicit substances, as well as the suspects' mobile phones and the Thai and Hong Kong currencies in their possession. The men were taken to the police station for questioning.

Officials also raided the suspects' room in the Makkasan area of the city and confiscated two kilogrammes of crystal meth. The suspects have been remanded in custody.

Police said they are interrogating the men to find out where they bought the drugs from, with fears Thailand is becoming a gateway for opium produced in the Golden Triangle region of the north, along with meth produced in Shan State in neighbouring Myanmar.

In June, officers seized a staggering 32 kilos or six million USD of heroin bound for Melbourne, Australia at the Bangkok Port.

In July, the Office of the Narcotics Control Board intercepted a 1.2kg Australia-destined heroin shipment declared as A3 inkjet paper at the Suvarnabhumi International Airport.

At the same airport, Thailand's Customs Department had seized 3.5 kilogrammes of heroin hidden inside parcels of pain-relief patches destined for South Korea, Hong Kong, and Australia this August.