At first glance, he could have been a participant in a postmodern art instalment.

But Victoria Police failed to see the artistic or the comical when an Irishman, aged in his 40s, wearing board shorts, a snorkel mask and flippers plopped his way across St Kilda Road on Thursday evening.

Pedestrians watched in bemusement as the man swam in the NGV moat.

He then began to snorkel his way around the moat outside the National Gallery of Victoria, much to the bemusement of staff, patrons and onlookers.

Fairfax Radio reported that the man is understood to have "caused trouble outside the gallery" on previous occasions.