A search and rescue operation led by Canterbury police in underway, after three men onboard a fishing vessel made a distress call at 4:30am.
The FV Jubilee was 12 nautical miles, about 22 kilometres, from Rakaia River mouth when it was last heard from.
Senior Constable Paul Beaver says the weather out on the water is severe, with high seas and winds making for trying conditions.
A sea and aerial search has been searching since 6:30am, and despite a raft and emergency locater being found, there is no sign of the crew or the vessel.
Two Coastguard boats, three commercial vessels, Westpac Rescue Helicopter and a fixed wing search plane are all assisting with the search.
The Police Dive Squad is also heading to the area to help.
Snr Const Beaver says police and rescue team are doing all they can to find the vessel and its occupants.
"Police are continuing the search for the crewmen but urge the public to stay away from the area due to the dangerous conditions in the area today."