A proposal by a group of telecommunications companies to build a broadband network would have a fundamental weakness if it was contingent on the involvement of Telstra, the competition watchdog said today.

Australian Competition and Consumer Commission (ACCC) chairman Graeme Samuel noted that the G8, as he referred to the group, had already indicated a detailed proposal might be available in the next week or so.

"At the present time, there are indications, in terms of public material, [that] suggest that their proposal relies upon some joint venture with Telstra," Samuel told reporters after addressing a Committee for Economic Development of Australia (CEDA) luncheon in Melbourne.

"That would have to be a fundamental weakness of that proposal because there's no way that any regulator can or should be able to require Telstra to enter into joint venture with other parties in the development of such a network.

"But if they were to come forward with a proposal that was not necessarily reliant upon agreement of parties other than those that have actually entered into an agreement with them -- in other words, not reliant upon Telstra -- then that might be an interesting proposal," he said.

Samuel earlier told the luncheon Telstra's plan to build an AU$3.4 billion fibre-to-the-node network was "not the only game in town" for faster broadband services in Australia.

Telstra has threatened not to build the network unless it gets regulatory approval on acceptable terms.

"There are others who are saying if this makes financial, commercial common sense, we may consider doing the same thing ourselves: rolling out our own fibre network," said Samuel.

"There is this group of eight significant telecommunications companies -- I call them the G8 -- that have indicated that they may well have the ability to put together a proposal to roll out a fibre network ... If they do, they will be subject to exactly the same rules as Telstra.

"They'll be entitled to a fair return on their investment and they will also have to provide fair access to other parties, including Telstra, to be able to compete in terms of the provision of telecommunications services.

"So the question of who actually rolls out the network is ultimately not a matter of concern to us," said Samuel.

The group of companies include number two telco Optus, AAPT, iiNet, Powertel and Macquarie Telecom.

Samuel also told the CEDA luncheon that the ACCC had received applications for immunity from several parties in relation to cartel activity.

"In the last 10 days -- the past two weeks -- we have received four applications for immunity in respect of significant cartel operations. In one case, there are three people standing in line waiting for immunity," said Samuel.

The ACCC grants immunity from prosecution to the first whistleblower on cartel activity if they are not the ringleader of the cartel activity and as long as they co-operate fully with the ACCC.