UPDATED 3.45pm: Pre-European human bones have been found under a water tank on Waiheke Island.

A forensic investigation was sparked when leg and pelvic bones were exposed in a trench on Palm Beach during building work yesterday.

Site manager Bruce Sciascia says he spotted a leg bone poking out, so called police who cautioned the area off and posted a security guard for the night.

He says police contacted him today to tell him the bones are pre European, and have been handed over to the Historic Places Trust.

He says it's the first time he's discovered human bones in the many years he's been working on Waiheke.