About 460 boats clear Customs at Opua each year but 90 per cent leave straight away. There is little space for visiting boats in the existing, privately developed marina, because all berths were sold when it opened in 2000.
FNH chief executive Andy Nock said about 30 berths in the expansion would be sold to "de-risk" the project, but the majority would be retained and leased to visiting boats.
If just 100 of those 460 vessels opted to stay instead of continuing to Whangarei or Auckland, millions would be spent in the Bay on repairs, maintenance, food and accommodation.
The number of berths had been reduced to accommodate increasing numbers of large catamarans sailing to New Zealand. The large berths could be used for two mono-hulls if required.
"Larger berths aren't widely available in marinas around New Zealand so we're aiming to catch as much of this growing market as possible," Mr Nock said.
A report commissioned by FNH put the financial benefit to Northland at about $23m a year by 2021. The Opua Business Association estimated the project would lead to 60 new jobs and a 44 percent growth in turnover.
The expansion has not been without controversy. Concerns have included its effect on tidal flows and water quality, public access, construction traffic on Opua's single access road, and the design of new on-shore facilities.
Local hapu vowed to fight the expansion in the Environment Court but eventually signed a Memorandum of Understanding with FNH instead. The agreement aims to protect the area's environmental, social, cultural and economic well-being.
FNH responded to criticism of the project's design by setting the buildings further back from the water and adding a picnic and performance area.
Chairman Ross Blackman said the plan was to transform the currently sterile, industrial-looking marina into a place families could enjoy.
FNH will also contribute to town improvement projects identified by the community group Love Opua, starting with a fitness trail suggested by the pupils of Opua School.
The new berths will be available from December 2016.
Mr Nock said the project was financed independently, not by ratepayers.