Emergency services were called about 9.50am to seafood restaurant Milano's on the Lake located on Pelican Marina in NSW's Hunter Region.
A NSW Fire and Rescue spokeswoman told news.com.au that part of the pier had collapsed and a portion of the two storey building had gone into the water.
"There were two occupants in the building at the time and they felt some movement so they obviously evacuated quite quickly," she said.
A spokesman for the lands division of the Department of Primary Industries told news.com.au that the agency had taken over management of the site in November.
"The previous leaseholder had not really managed the site well," the spokesman said.
After the department took over, the spokesman said site inspections were done which identified some maintenance issues. The department has since replaced decks, hand rails and performed electrical works.
"But there was no indication this would happen," the spokesman said of the marina collapse.
"There was no forewarning, we are trying to determine what happened."
But Ben Ensor, the former director of Elidock Pty Ltd, which is now under external administration, told the Newcastle Herald in 2012, that delays to dredging Swansea Channel had left it blocked and this had changed the water flow.
The marina had lost two berths and another two berths were on the brink of collapsing, he said at the time.
The state government's dredging was criticised at the time for being too small, and blamed it for the collapse of the Belmont Airport seawall as well as the marina berths. In 2014, a major $2.5 million dredging operation was announced for the channel.
The marina will be closed until the department can assess the cause of the collapse.
In addition to the restaurant, the spokesman said there were three other tenants at the wharf who were renting out office space.
Authorities have set up an exclusion zone around the restaurant, have secured gas and energy and placed floating booms in the water to ensure any pollutants don't spread further into the waterway.
NSW Police is asking for sightseeing boats to remain clear of the incident site, as the wake is creating issues for emergency services.
In a Facebook post the restaurant said: "Everyone is ok, just here with emergency services now".
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