The man was one of four people in the 18-foot catamaran which overturned at popular fishing spot Dutton Way after 11.30am Wednesday.

Emergency response controller Gavin Slade, who is also an acting senior sergeant, said police were alerted after several calls from the public to Triple Zero.

Acting Senior Sergeant Slade said Water Police, Coastguard, local police and the HEMS 4 air ambulance responded. 

The police air wing flew Water Police investigators and divers to Portland from Melbourne to help with the rescue efforts. 

Acting Senior Sergeant Slade said he believed the three other people on the boat were a group of friends, possibly some family members and that they weren't local residents, but the exact details were yet to be confirmed.  

"Three are in reasonable condition at the Portland hospital and they're assisting police with our enquiries at the moment." 

Portland resident Anne McCurdy said she was out looking for whales when she saw police arrive.

"I pulled over to see what was happening and someone said 'someone is missing'," Ms McCurdy said.  

Ms McCurdy said she watched as one person on the upturned boat and two people were rescued from the water about 300 metres from where she was standing. 

"They dropped a guy from the HEMS 4 chopper down and he swam to the boat to do medical checks on the guys that had been rescued." 

She said the three people were taken onto the Portland Coastguard boat where they remained for about 30 minutes before they were brought to shore. 

In other news a deceased man was found in a burnt-out car at Branxholme on Wednesday afternoon. 

Detective Sergeant Mark James from the Hamilton police Crime Investigation Unit said there were no suspicious circumstances and police were investigating. 

He said Ardachy Estate Road was closed to traffic on Wednesday and would remain closed until Thursday afternoon while police conducted their investigations and asked for motorists to avoid the area.