The state weather service said it had sought the help of police and local governments to help guard the rest of the sensors, which it suspects are being cannibalised by poor locals who sell them for scrap and quick cash.

A million people have been left without advance warning of flash floods after the theft of the sensors at Agno river and its Tarlac river tributary, said Ranny Ragasa, a member of the technical staff at the weather service.

"When there are heavy rains, 30 towns could easily go under water," said Ragasa, who mans the weather service's monitoring centre on the bank of the Agno.

Ragasa told AFP by phone that thefts of grounding rods for the sensors had occurred at five stations of the Agno and Tarlac since December.

"It's possible they sold the metal for scrap, not knowing how important these are to the operation of our stations," he said.

"These systems are worth millions of dollars," he said. "Repairing them would cost the government thousands (of dollars), but they might have sold for 200 pesos ($4.50) a kilogramme (2.2 pounds) at the scrapyard."

"If you're living in poverty, the temptation to steal is always there."

At least half a million people were affected during the last major flooding of the Agno basin during the height of Typhoon Parma in October 2009, Ragasa said.

Parma killed more than 400 people, according to government data.