Assuming its estimated diameter of over 300 km is accurate, it may be the fourth- or fifth-largest known moon of a Trans-Neptunian object, after Pluto I Charon, Eris I Dysnomia, Orcus I Vanth, and possibly Varda I Ilmarë and Salacia I Actaea.

In spite of its relatively large size, Hiʻiaka is not in hydrostatic equilibrium because its elongated shape is inconsistent with that expected for its current rotation period.

[2]: 163–164 The near infrared spectrum of Hiʻiaka is dominated by water-ice absorption bands, which means that its surface is made mainly of water ice.

The presence of the band centered at 1.65 μm indicates that the surface water ice is primarily in the crystalline form.

Currently it is unclear why water ice on the surface has not turned into amorphous form as would be expected due to its constant irradiation by cosmic rays.