Telstra spokesperson Stuart Gray said the cable that connected the ISP's Australian customers with Asia, Europe and the Middle East, SEAMEWE3, was cut at the bottom of the ocean near Singapore around midday yesterday.
The cable, which is owned by a consortium of around 90 companies worldwide, including Telstra, holds around 60 percent of the ISP's international Web traffic.
Gray said it was not known when the cut would be repaired.
-That's going to take a couple of days. We've got to get a ship out," he said.
Gray said the cause of the damage was not known. He said it possible the cable was cut accidently by a deep-sea fishing trawler or by an -undersea earthquake". It was unlikely the damage was caused by any malicious parties, he said.
Gray said Telstra had already taken steps to redirect most of its international traffic through other networks, and was running at 75 percent its normal load capacity.
He said that meant users who logged onto international sites during off-peak times would not experience any serious slowdown.
However, -people logging onto international sites (during peak times) may see some slowness logging on," he said.
The busiest time for the ISP was the evening, he said.
Users logging onto Australian-hosted sites should remain unaffected, he said.
The ISP ran at 30 percent its normal capacity yesterday afternoon.
