Ran the JOSM validator over this section of map:
http://opengeofiction.net//#map=17/-8.4501/171.8488
And got 13 errors related multipolygons, affecting these items:
relation 74797 relation 74796 relation 74799 relation 74798 relation 74801 relation 74800 relation 74672 way 8175720 way 8175721 way 8175722 way 8175717 way 8175718 way 8175719
And I have no clue how to begin fixing them. If someone could clean this up into something resembling sensible data, it would be appreciated.