Phase two of the rebuild - in consultation with Heritage NZ and specialist architects - would see the rear of the hotel transformed to mirror the front but with extra rooms instead of the restaurant.
The couples' aim was to have the Duke ranked among New Zealand's top five boutique hotels, alongside the likes of Eichardt's Hotel in Queenstown.
The Duke's restaurant was already in the top 20 in the country for quality and turnover, serving 150,000 meals a year or 1400 a day at the peak of summer, Mr Kinnaird said.
The couples had also created a new restaurant on Paihia Wharf, Charlotte's Kitchen; bought the former Triton Suites on Wellington St, turning it into the Duke Motel; and bought land on York and Church St, allowing then to shift parking off the prime waterfront site, build a gym and commercial laundry, and solve their most pressing problem, staff accommodation.
They also planned to turn the Masonic Lodge into backpacker-style accommodation for staff.
The Duke employs 70 permanent staff and 140 in peak season. By this time next year it would have 140 permanent staff and 210 at peak.
Mr Kinnaird said a function at the Duke last weekend was a way of celebrating the transformation as well as thanking supporters.
"This didn't happen with just the four of us, it happened with a huge amount of support from the local community, the banks, our suppliers, Harnett Builders and locals who built the extension and 20 bathrooms in eight weeks ... It's a chance to pat everyone on the back and say well done."
The party also marked 190 years since the first hotel on the site, called Johnny Johnston's Grog Shop in what was then lawless Kororareka.
Mr Johnston, a reformed convict, later changed the name to the more upmarket-sounding Duke of Marlborough.
In 1840, with ink barely dry on the Treaty, the Duke was granted New Zealand's first liquor licence. The first Duke of Marlborough burnt down in the Northern Wars of 1845-46. At least one of its successors also went up in flames.
Today's Duke began its life around 1900 as an accommodation block at Cable Bay for operators of New Zealand's first underwater telegraph cable. It was barged to Russell about 1932.