Authorities in Concord have sealed off one of the town’s water storage tanks and issued a ban on outdoor water use after they discovered a broken lock on the tank’s access hatch, police said.

The water division of the Concord Department of Public Works is conducting tests to ensure that the water has not been contaminated but said it had sealed the facility off from the rest of the water supply “out of an abundance of caution.’’

Police said the water flowing into residents’ homes remains safe to drink.

A water department employee discovered the compromised lock during a routine site visit Monday morning, said Concord Police Chief Joseph O’Connor. The tank is located just off Route 126 near the Walden Pond Reservation.

“We don’t have any indication that anyone got through the hatch,’’ O’Connor said. “The lock was broken off, but we didn’t have any other indication that someone actually entered the hatch.’’

Because the closure has temporarily reduced the town’s water supply under already dry conditions, residents are being ordered to conserve water by curtailing outdoor use.

“Due to regional drought conditions combined with this significant water system limitation, customer demand has reached the limit of our available water supply,’’ the Police Department wrote on its Facebook page. “Immediate curtailment of all non-essential water use is necessary until further notice.’’

The director of the department was not available Tuesday to say how long the tests — and the water restrictions — would last.

The tank, which holds 5 million gallons, is one of two storage reservoirs that store water that is collected from the town’s six water sources but is not used right away, according to Town Manager Chris Whelan.

Whelan said initial test results on the water in the affected tank are expected Wednesday, but more complicated analysis may take longer.

The water restrictions may be in place for a few days, Whelan said. He added the restrictions may also be weather dependent.

Whelan said the town is optimistic that the water is not contaminated because police did not find any evidence anyone had opened the doors to the tank.

“We’re confident that there was no intrusion into the water system, but we discontinued it from the water system as a precautionary measure,’’ Whelan said. “We’ll get the tests back tomorrow, and we’re confident they’ll come back clear.’’

The list of water restrictions includes a ban on activities such as watering lawns or filling swimming pools. The advisory also instructs residents to reduce their indoor use as much as possible.

The restrictions are enforceable by fines of $50 for first-time offenses and $100 for additional ones.

A spokeswoman for the Massachusetts Water Resources Authority said the agency is helping conduct the tests but only in a “neighborly way,’’ as the underground reservoir is not the authority’s facility.

Ed Coletta, a spokesman for the Massachusetts Department of Environmental Protection, said the state agency has been notified of the closure and is monitoring the situation. The department is not involved in the water testing.

Dylan McGuinness can be reached at dylan.mcguinness @globe.com. Follow him on Twitter at @DylMcGuinness.