Australian Prime Minister Julia Gillard has asked the authorities in Pakistan to investigate and explain the brutal killing of 21,000 Australian sheep in Karachi.

The decision to cull the sheep was taken after sheep tested positive for salmonella bacteria, which can also infect human beings. The ship had arrived in port city of Karachi two weeks after the authorities in Bahrain did not allow the ship to unload sheep in the country due to health concerns.

Australian shipping company, Wellard Rural Exports had claimed that Pakistani veterinary authorities had cleared the shipment after conducting quarantine checks. However, officials in Pakistan later said that the sheep are infected and must be culled.

Gillard said that she has asked Pakistani leader Raja Pervez Ashraf to look into the matter when she met him during a summit of European and Asian leaders in Laos.

"I did raise with the prime minister of Pakistan my concern about the graphic and very cruel images we've seen of the treatment of Australian sheep. I explained to him that Australians are distressed to see these acts of cruelty and that I wanted the matter investigated," she said.

The Ocean Drover had left Australia with 75,000 sheep over a month ago and has already unloaded 53,000 of them at two other ports until officials in Bahrain found that some of the sheep were infected with "orf" or scabby mouth disease. The case has sparked an angry response in Australia and from animal activists from around the world. Images of their brutal slaughter were aired by ABC's Four Corners programme on Monday in Australia.