Construction of the Elizabeth Quay inlet was temporarily halted after a dolphin nudged its way under a protective net surrounding the Swan River works.
Officers from the Swan River Trust and Department of Parks and Wildlife were at the site but were not concerned for the animal’s welfare.
The submerged net, known as a silt curtain, is in place as part of works to open up the inlet, which involves building a limestone and granite wall to hold back the water and allow excavation.
A spokeswoman for the Metropolitan Redevelopment Authority, which is running the $440 million State Government project, said the silt curtain had been opened to allow the dolphin to swim away.
The dolphin swam free and work resumed.
Construction on land was not been affected.Dolphins on occasion use Swan River walls to catch fish and it is believed the animal may have got close to the silt curtain for the same purpose.