Mr Grant said the extension would allow for 150,000 additional passengers to the 400,000 yearly total, but extra boats or cruise ships were not MDA's concern.
"It would allow for more passengers in the sense it will make the movements of the boats a lot quicker, but there is no proposal there will be more boats."
The new basin, built from existing rock and rocks from the Cleddau River 2km away, would enable existing boats to park in the fresh-water basin rather than the nearby deep one. The project involved moving the existing breakwater and dredging to enlarge the harbour, realigning the current wharves and replacing them with floating wharves and redecking/extending the visitor terminal.
During the summer season the harbour is one of the busiest in New Zealand, with up to 120 vessel movements a day.
Mr Grant said the upgrade had been six years in the planning and would "ensure Milford Sound maintained its reputation as New Zealand's leading tourist attraction".
"Essentially we are just shifting the breakwater," he added.
Stockpiling of rocks for the reef would begin next month, and construction would start in March.
Mr Grant said he expected the harbour to be completed by August.
The project was the most significant investment by the MDA since the original tourism development from 1989 to 1991.
The $6 million would come from the MDA levy, which is funded through companies that used the fresh-water basin.
- OTAGO DAILY TIMES