A tourist's holiday skinny dip turned sour after he was reportedly bitten on the penis by a venomous spider.

The 22-year-old Canadian fell asleep in the sand dunes after swimming nude at a beach in New Zealand's far north.

A female katipo spider. Credit:Kirk Hargreaves

He woke to find his penis swollen and painful, with a bite mark on the shaft, news agency NZPA reported.

The case was covered today in the NZ Medical Journal after the man recorded the first known case of myocarditis, or heart inflammation, caused by a spider bite.