A Wellington marine centre lost over half of its marine life in a devastating act of vandalism overnight.

Arriving to the centre at Wellington's historic bait shed at about 10am, Island Bay Marine Education Centre discovery programme manager Julian Hodge found the water system had been dosed with chlorinated detergent, poisonous to many of the marine species in the tanks.

''I found a whole load of bubbles coming out of our tanks and gutters and a whole lot of our fish were dead, and many were in the process of dying.''

While immediate attempts were made to move distressed fish to uncontaminated water and centre staff, volunteers and the Fire Service were called in to flush out the tanks with fresh sea water, many marine animals were lost or had to be euthanised.

''We expect over the next two or three days, there will be quite a few more casualties as well.''

The centre's larger animals, plus the starfish, seahorses, and crabs, mostly survived though others, like the pipe fish, had not.

''We've lost fish here that have been with us for 16 years.''

Mr Hodge said he doubted the act, which killed between 70 and 80 fish, was a silly prank.

''It's a deliberate act, someone has deliberately poisoned the animals in the aquarium.''

The marine centre's large ship's anchor chain was also taken as part of the incident.

Mr Hodge said the vandalism saddened and infuriated him, not just for the centre staff such as him, but also for the community volunteers who gave so much of their time to the aquarium.

''It's absolutely devastating - very, very upsetting.''

''Our focus now is making sure all the existing fish are alive and healthy and in the best possible environment.''

He said the centre would also aim to re-open to the public on January 6 as planned.

''We're not going to let someone doing something like this get in the way.''

The act had been reported to the police, who would be interested to hear from members of the public who had any information on the incident.