Port to Port Cruises and Wildlife Tours owner-operator Rachel McGregor watched the squabble last Friday along with passengers from her boat on the harbour.
The tour had been heading out and paused at Harington Point to watch a fur seal "tearing up the beach".
"We couldn’t see what it was running at until the fur seal came running out of a cave," she said.
"There was a bit of roaring from the sea lion ... [the fur seal] looked absolutely petrified."
Despite the hostilities, the animals were all bluster. After staring each other down, the fur seal slunk away to another corner of the beach and the sea lion eventually returned to the water.
"I’ve been running these trips and working on the ferry for about 20 years and I’ve never actually seen a sea lion do that to a fur seal.
"You don’t normally see them interacting ... and definitely not that full on," she said.
