There is no easy fix to this. It will be rusting from the underside, and will have spread through. To fix it properly would require removing the petrol filler pipe and flap. Cleaning up all the rust and repainting both sides before sealing with some good quality non hardening underseal before refitting the petrol flap. Access underneath would be awkward. Its a well known probably caused by the fact Honda did not fit a wheel arch liner. And the Jazz is not the only car effected with this problem.
