For Nelson itself, the cancellation due to Tuesday night's snap national lockdown was also a major blow, said mayor Rachel Reese.
She cited a report from the Nelson Regional Development Agency that the event was worth around $400,000 to the regional economy.
"The seafood industry is at the heart of who we are as a community, so it's a real celebration of achievements of the industry and we are very proud to host that conference.
"[Seafood] is part of our regional economy but this one was important not just because it was an event with a lot of attendees which supported our accommodation and hospitality sector, but because it brings together an industry so important to our region."
Hamilton said Seafood NZ had started working towards the conference a year ago.
The cancellation was "devastating".
"Unwinding something as big as the Seafood New Zealand conference is not easy. Flights and accommodation must be cancelled, venue security, audio-visuals, photographer, media, and sponsors must all be dealt with."
And then there was the matter of all the food.
This meant for some Nelson-Tasman residents, the conference cancellation had a bright side.
Food for the delegates, some of it provided by New Zealand companies attending the conference and some prepared by the Rutherford Hotel Nelson's executive chef, Jeff Scott Foster, was donated to people in need through the Nelson Environment Centre Kai Rescue programme.
While much of the seafood intended for the conference and cocktail function on Friday night was stopped in transit and returned, hundreds of kilograms of seafood, vegetables, fruit, snacks and bread was given away.
Seafood NZ chief executive Jeremy Helson said he was deeply disappointed the conference could not go ahead due to Covid-19 but the decision was inevitable.
An enormous amount of work had gone into the conference, but the collaboration between the Nelson Regional Development Agency and the Rutherford Hotel had ensured none of the food went to waste, he said.
"Without their work, the mammoth effort of redistribution would not have been possible."
The Kai Rescue programme was established in 2017 Nelson Environment Centre with the aim of minimising food waste in the community.
The centre partners with more than 60 food recipient organisations from the Nelson Tasman region who distribute food to individuals and families in need. Kai Rescue is run by volunteers.
Helson also acknowledged the seafood companies who agreed the kaimoana should be redistributed to those who would benefit from it.
"These are difficult times, and some in the community are having a more difficult time than others. If this puts a smile on a few faces, we will be happy."
Seafood NZ was now looking at options for rescheduling the event in Nelson at some later date.
The $2 billion New Zealand seafood export industry employs 13,000 people full time.
Last year it harvested more than 450,000 tonnes of seafood, not including aquaculture products.