A Japanese school has been hit with a $27,000 water bill after a teacher in charge of pool maintenance left a tap on for months hoping to stop coronavirus infections.

The teacher, who has not been identified, thought a constant flow of fresh water into the pool would keep it Covid free and left the tap on from late June to early September.

Normally, chlorine and filtering machines maintain the pool water's quality, "but the teacher somehow got the wrong idea that pouring new water in would also do the trick and even help prevent Covid," local education board official Akira Kojiri told AFP.

Other staff members occasionally noticed the running tap and switched it off, but the offending colleague soon switched it back on.

As a result, Kojiri said, an estimated 4,000 tonnes of excess water was used in just over two months -- enough to fill the pool 11 times over.

Local authorities in Yokosuka in central Japan's Kanagawa prefecture are now demanding the teacher and two supervisors pay half of the 3.5 million yen ($27,000) bill.

"We deeply apologise to our residents for causing (financial) damage to our city," Yokosuka authorities said in a statement.