TELSTRA will bump its cable internet service to a theoretical maximum of 17Mbps, opening a pipe for its forthcoming movie download service.

Telstra will charge an extra $10 per month for the speed boost, which more than doubles the current 8Mbps limit.
Both the new cable and movie download services begin next month.

Telstra said trial users of the new services, branded "Cable Extreme", had experienced speeds of up to 17Mbps, but cautioned that real-world performance was affected by location, system configuration and network traffic.

"We have been upgrading the cable network's infrastructure and we have been trialling software changes to evaluate the network's ability to provide even higher speeds," BigPond group managing director Justin Milne said.

Rolled out in competition with Optus in the 1990s in a rush to dominate the then new cable TV market, Telstra's cable network, which it shares with Foxtel, passes 2.6 million homes.

But Telstra has previously said it had no plans to extend the network to new areas.