Beachgoers in Melbourne have been treated to the “cosmic and magical” sight of bioluminescent algae off St Kilda beach this week.

Richard Pensak, a marine biologist at local environment group Earthcare St Kilda, spotted the bright pink-coloured cloud in the water on Sunday, and immediately knew what it was.

When he returned to the water after dark, crowds had already gathered to see the “really pretty and sparkly” spectacle.