The aircraft had been parked in a recently opened grass parking bay. Whilst the aircraft was taxiing from the parking area onto a taxiway, the aircraft's left main landing gear wheel sank in soft sand on an unmarked irrigation outlet. The metal plate covering the outlet was not supported on one side. The wheel ran onto the unsupported side of the plate, causing it to flick up and puncture the underside of the wing and fuel tank. Exit routes from the parking area were not marked and the path taken by the pilot was not normally used. Local Safety Action The airport authorities are intending to clearly mark parking area entry and exit routes.