Chathams and offshore island coastlines are under island_poly.
typically tagged as natural=coastline

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Check for errors here:
and/or here:

Coastline should be continuous and keep water in its right side, so go around an island in a counter-clockwise direction.

Elevation is always 0, so skip it.
HB: line reversal added for (at least) Stewart Island
(old) what do the a1016 and a1020 attributes contain?
HB: coastline for stewart island uploaded 20 Dec 2010.
HB: coastline for catlins uploaded 31 Dec 2010 in changeset 6816775.
HB: coastline for otago uploaded 14 Jan 2011 in changesets 6973706, 6973727, 6975789. chch to kaikoura uploaded in #7015964
HB 22 Aug 2012: re-approved for the mainland with no changes.
SD 23 Aug 2012: this was split up into smaller <1000 feature sections.