The Agriculture Department has blocked the marketing of about 30 million pounds of Australian beef because it contains illegally high pesticide residues, and is threatening to cut off Australian beef entirely unless the problem is brought under control.

The department's Food Safety and Inspection Service said yesterday that it took the action after inspectors found impermissible levels of pesticides, including DDT and dieldrin, in samples of western Australian beef tested earlier this month. It was the eighth such violation this year, and the third since May 25, when Australia toughened its inspection procedures in response to U.S. complaints.

As a result, department spokesman Nancy Robertson said, "We are not permitting the entry of any Australian beef processed before May 25."

Most Australian beef arrives here frozen and is used in canned beef products and as hamburger. Australia exports about 678 million pounds of beef to the United States each year, accounting for 29 percent of U.S. meat imports and nearly 8 percent of domestic beef supply.

Robertson said Australian authorities estimate that about 30 million pounds of beef processed before May 25 is already in the pipeline, either on ships bound for the United States or in warehouses awaiting distribution. The Agriculture Department is allowing the beef to be unloaded and stored, she said, but has effectively blocked marketing by refusing to inspect the meat.

The action does not affect beef processed since May 25, but Robertson said the department intended to step up its sampling program for Australian shipments. Shipments of beef processed since May 25 "are just starting to come in now," she said.

DDT and dieldrin were banned in the United States in the 1970s, and many other countries have banned or restricted the chemicals because they persist in the environment and accumulate in animal tissues.

Robertson said the Australian shipments contained DDT residues as high as 103 parts per million, more than 20 times the U.S. legal limit. She said Australia does not allow DDT for use in livestock production, and officials there have been unable to explain the high levels.

"No one really knows for sure," she said. "They only had one violation last year, so whatever controls were in the system have broken down."