The high court certified a class covering about 72,000 former and current Wal-Mart workers. One legal expert said the decision "isn't good news for Wal-Mart".

"My speculation is that a jury is likely to find for the plaintiffs, given New Jersey juries and the pretty strong evidence put on elsewhere," said Carl Tobias, a law professor at the University of Richmond in Virginia. Wal-Mart, based in Bentonville, Arkansas, faces more than 70 US wage-and-hour suits, including class actions by employees claiming the company failed to pay for all hours worked or didn't compensate them properly for overtime.

Since December 2005, juries in Pennsylvania and California have awarded Wal-Mart workers a total of $US251 million ($A303 million) in pay and damages over such claims.

"We're disappointed with the decision and we're studying the opinion," Wal-Mart spokesman John Simley said.

Workers' lawyer Judy Spanier said her clients were "very pleased" with the decision. "It essentially adopts every argument we made," she said.

The ruling sends the case back to state court in New Brunswick for pretrial evidence-gathering.

The trial court first refused to grant class-action status, saying the case would be unmanageable. A mid-level appeals court upheld the decision. The Supreme Court found both lower courts were in error.

The workers claim Wal-Mart forced them to work through meal breaks, locked them in retail stores after they clocked out and coerced them into working off the clock.

The New Jersey action class will cover current and former hourly Wal-Mart staff employed from May 30, 1996, to the present.