The building, though never fully completed, was opened by Rear Admiral Charles S. Sperry, commander of the Great American White Fleet, on August 13, 1908.
Present US ambassador William McCormick will be guest of honour at a gala dinner on Wednesday - 100 years to the day since the opening.
Work is also under way on an extension to the Bath House's north wing, part of the original plan but never built when the project ran out of money. A larger addition to the south wing is scheduled for construction next year, also to complete the building to original specifications.
It is hoped the extra space will allow the museum more room to display local Maori taonga, and facilitate their loan from other institutions nationally and overseas.
Museum director Greg McManus is particularly keen to see the return of a 3.5m model canoe by carver Tene Waitere which was gifted to the Duke of York in 1901 and is now in storage at the British Museum.
The Bath House was built entirely of native timbers, including kauri and puriri. Totara logs used for some internal pillars were so big they had to be shipped to England for turning and then shipped back to New Zealand.