The police constable confused New South Wales in Australia with South Wales in Britain as he brushed up on his law knowledge when using Google.
But as a result, the shopkeeper, who is located in the South Wales valleys, was mistakenly warned he could be arrested for selling a pipe-smokers' "bong" that is generally used for smoking marijuana.
The Pc, who has not been named, had showed the shopkeeper an internet printout on the law's stance about selling such equipment, which he stated proved it was an illegal act in Wales.
But the unidentified retailer later discovered the law referred to Australia's most populous state, home to the country's biggest city Sydney, more than 10,000 miles away.
The bobby was left red-faced after senior officers told him he was wrong. Police chiefs were later forced to apologise for the embarrassing blunder.
Martin Palmer, who supplies the Savers Xtra store in Bargoed, near Caerphilly, South Wales, said the "bong" smoking tools had always been legal in Britain.
"The policeman went into the store and told the shopkeeper what he was selling illegal," said Mr Palmer, a 52 year-old company director.
"He quoted a law and gave him a piece of paper with the relevant clause on it.
"But he phoned me up asking for advice - and I've been doing this for 25 years so I knew it was OK."
Mr Palmer, of wholesalers Avalon based in Portsmouth, Hants, added: "So I searched the act and this section on google and found that it was a law in New South Wales in Australia."
The PC showed a document titled "Misuse of Drugs and Trafficking Act 1985" and drew the shop keeper's attention to Section 11A.
It states: "It is an offence to sell, supply or display for sale a bong or ice pipe, or the component parts of a bong or pipe, whether or not the bong or pipe was intended to be used to administer a prohibited drug."
The legal crackdown in Australia was introduced to deter people from using the equipment for smoking cannabis instead of tobacco. It is not outlawed in Britain.
Police in Gwent, South Wales, apologised to the shopkeeper.
A spokesman said: "The officer in question was acting in good faith but on this occasion we got it wrong.
"The local inspector has visited the shopkeeper and apologised personally for the confusion and any distress caused."
