A shark attacked a surfer at Sydney's famous Bondi Beach Thursday, seriously injuring the man's left hand and arm, Australian police said.There has not been a shark attack at Bondi for decades.The attack comes one day after a navy diver was attacked by a shark in Sydney Harbor. The diver lost a hand and has severe leg injuries.The shark attacked the 23-year-old surfer just before dark."He said he felt a tug on his arm. When he looked down he saw a bad bite on his left hand and arm," a police spokesman said."He yelled for help and nearby surfers assisted him. Some off-duty doctors on the beach helped stem the bleeding."There have been at least four other shark attacks so far this Australian summer, one of them fatal, sparking a global media frenzy of "Jaws" proportions. Last month, there were three shark attacks in Australian waters in just two days.The last fatal attack at Bondi was in 1929.Wildlife officials say Sydney's cleaner ocean water, due to offshore sewage outlets and stormwater cages, has encouraged marine life closer to shore, which in turn has attracted sharks.Wednesday, the navy diver punched a shark as he was being attacked in Sydney Harbor, fending off the predator but suffering severe injuries to his right hand and leg. Shark nets are used off all of Sydney's ocean beaches.They are rectangular pieces of net suspended in the water between buoys. When sharks get caught in the nets they drown.The nets are not intended to form a complete barrier and sharks can still get through. The nets act as a deterrent by interrupting the territorial swimming patterns of sharks.