An Australin builder was bitten by a venomous Redback spider on his penis while on a building site toilet.

The 21-year-old was using a portable toilet at a property in the southern Sydney suburb of Kogarah.

Paramedics were called to the scene but the man was able to make his own way to the emergency department at nearby St George's Hospital.

Doctors have said the young man is in a stable condition and is expected to be shortly discharged.

Generally nocturnal, Redback spiders tend to live close to human homes where they eat insects and smaller spiders.

Professor Julian White from Adelaide’s Women’s and Children’s Hospital said: "Going back 80 years or so when people were still using outhouse toilets it was extremely common, something like up to 80 per cent of cases of spider bites were bites on the male genitalia.

"Typically they were using the toilet. But it’s much less common now, I can’t think of a case.

Anti-venom for the Redback bite was first developed in the 1950s.

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Doctors advise those bitten to apply an ice pack or cold compress to the bite and contact emergency services.

"Professor White added: "It’s our fault that redbacks are on the dunny[toilet], in fact we’re the best thing that ever happened to them — where there’s humans, there will be insects for them to feed on.