"I don't like spiders all that much, I must confess," Nick Le Souef said as he embarked on his record-breaking attempt on Monday.
"I'm not afraid of them but I don't like them particularly."
Mr Le Soeuf is hoping to raise about $50,000 for children's charity Variety.
The 67-year-old had originally hoped to do the stunt with snakes, but licensing issues put a stop to that caper.
So instead he's spending his days and nights with potentially dangerous redbacks and bird eating spiders, black house spiders and the common huntsman, plus a few of the white-tailed variety.
There were about 400 of them until an "arachnid massacre" when the creatures started eating each other.
A few crickets and cockroaches will be thrown into the window of Mr Le Souef's Flinders Lane opal shop to keep the remaining 300 happy.
Mr Le Souef's not afraid of being bitten or anything crawling over him while he sleeps in his swag, but hopes the "fear factor" for others will encourage donations to Variety.
He has a history of these stunts, holding Australian records for three-week stints in a snake pit, a shark tank and also in a cage with redback spiders.
Mr Le Souef doesn't think he's crazy but former AFL footballer Peter `Spida' Everitt, a Variety supporter, does.
"No way known I could sit there with four or five hundred spiders, not for five minutes I reckon," Everitt said.