The donation worth $3.6 million was made in November by a person who a month earlier had already given $3,300 in cash for the municipal waterworks, Osaka Mayor Hideyuki Yokoyama told a press conference.

"It's an absolutely staggering amount," said Yokoyama, adding that he was lost for words to express his gratitude.

"I was shocked."

The donor wished to remain anonymous, the mayor said.

Work to replace water pipes in Osaka, a city of 2.8 million residents, has hit a snag as the actual cost exceeded the planned budget, according to local media.