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TOKYO, Sept 29 (Reuters) - Japanese imports of U.S. frozen beef fell by more than a quarter in August after Tokyo hiked tariffs on such shipments as it tries to protect local farmers, data from the country's agriculture ministry showed on Thursday.

But that helped prompt a nearly 55-percent surge in shipments of U.S. chilled beef as buyers looked for substitutes, meaning overall imports of the meat from the United States were up by around a fifth.

Japan said in July that between Aug. 1 and the end of March next year, tariffs on frozen beef imports from the United States and other countries would jump to 50 percent from 38.5 percent, as a "safeguard" mechanism was triggered to help domestic producers.

Imports of U.S. frozen beef slid to 4,317 tonnes in August, down 26 percent from a year ago, the trade data showed.

Japan's import of chilled beef from the United States surged 54 percent in August from the year before, boosting overall U.S. beef shipments by more than 20 percent to 18,038 tonnes.

Meanwhile, August imports of frozen beef from Australia rose 30 percent to 15,010 tonnes as some buyers looked for an alternative source for frozen meat. (Reporting by Yuka Obayashi; Editing by Joseph Radford)