Reef Watch Victoria program co-ordinator Wendy Roberts, of the Victorian National Parks Association, said four juvenile fish had been reported this summer in the southern part of Port Phillip Bay.

''The fact that we're seeing them now is very exciting,'' she said. ''It's like a rare bird reappearing in the forest; it's very unusual.''

Museum Victoria ichthyology senior curator Martin Gomon believes the blue groper's presence in Port Phillip Bay has not been documented before.

''For them to have reached Victorian waters, the populations are increasing to the point that the larvae are able to reach us [by the wind and tides],'' he said. ''It's indicative that the population is increasing.''