The thin white line is possibly a camera aberration. It can be seen on some similar shots here https://inaturalist.ala.org.au/observations?place_id=6744&preferred_place_id=6744&quality_grade=research&taxon_id=785542
Thanks. May be just natural variation - the iNtat sightings seem to vary in other ways as well, like how yellow the moth is, how strong the black markings are ...