Tin Shui Wai - one of the city's poorest areas - has the highest grocery store food prices, with pork and beef prices more than 50 per cent higher than in other districts, a Labour Party survey has found. The citywide study also found that the average prices of many common foods have more than doubled since 2007. That far outpaced the 18 per cent growth in public housing residents' average income over that period. Releasing the findings yesterday, Tam Chun-yin, the party's secretary and community officer, called for the government to build public market buildings to break up the monopoly interests controlling Tin Shui Wai's retail sector. He also called for the introduction of a salaries tax credit to subsidise people earning below a certain amount and help them survive amid high inflation. "We believe the exorbitant food prices in Tin Shui Wai are caused by a lack of market competition," Tam said. "The government should build public markets there as they do in other places." In Tin Shui Wai, except for the two major supermarket chains, most grocery stores are located in the six markets managed by the listed group Link Reit, a real estate investment trust. In some other districts, shoppers can choose between market buildings - managed by the Food and Environmental Hygiene Department - and street markets. The survey, conducted over three weeks in January, covered 30 markets and 19 supermarkets across the city. It looked at the prices of 16 food items including rice, pork, beef, chicken, fish and vegetables. It then compared the average prices to prices recorded by the government in 2007, noting that day-to-day fluctuations could occur. It found pork prices in Tin Shui Wai were 57 per cent higher than the average price in other districts. Beef sold for HK$222.50 per kg, 51 per cent higher than the average price of HK$146.50 in other districts. Comparing the January prices recorded by the party and the 2007 official data, the average per-kg price of beef brisket has risen by 110 per cent; that of grass carp by 111 per cent. The price of rice has gone up by 94 per cent. Citing Census and Statistics Department figures, Tam said median income in public rental households rose by 18.2 per cent - to HK$13,000 - between 2007 and the third quarter of last year. The party also called for a yearly review of the minimum wage, to better link income growth to inflation. The Food and Health Bureau is studying whether competition in the supply of beef from the mainland would slow price rises.