Hong Kong parents who feed their babies formula have more to worry about than just the products' health effects - they find themselves in a battle with mainland visitors just to buy it. Shortages are common, especially during national holidays, when visitors often arrive. The problem arose in 2008 after melamine, an industrial chemical, was added to mainland milk supplies. That poisoning left at least six children dead and hundreds of thousands ill. The competition for formula has become a major source of tension between Hongkongers and mainlanders. Recent reports suggested the problem had spread internationally - with newspapers as far afield as Germany and Australia reporting that Chinese buyers had cleared shop shelves of baby formula. "Mainlanders tend to have a misconception that the Hong Kong product is more guaranteed," said Charles Urbain, a spokesman for formula maker Mead Johnson Nutrition. He said Hong Kong was a unique market as much of the supply went to meet the massive demand of mainland parents, making it difficult to plan the delivery of supplies, which were imported from the Netherlands. The company moved to meet mainland demand last year by opening a factory in Guangdong. "The quality of our product in China is exactly the same as in Hong Kong," Urbain said. "What is different is that the packaging is in simplified Chinese characters on the mainland." He said Hong Kong would continue to receive its supply from the Netherlands. Still, mainland parents prefer the Hong Kong packaging, making it popular with parallel traders, who buy tins of formula in Hong Kong and then sell them across the border, avoiding the payment of import taxes. One trader said they were paid HK$10 for every tin of formula they brought across the border, meaning they earn HK$100 for each of the typical 20 trips they make each day.