I had the same thing from a guy claiming to be "John Mathews" with a heavy Indian accent. When I refused to log onto their non-telstra site they eventually told me to F*** off and that my internet would be disconnected the next day. Later I got a call back from another person with a heavy Indian accent claiming Telstra was going to disconnect my internet tomorrow. When I informed them I was with Optus they hung up. There are details on this scam on the Australian government website - ACCC site the scamwatch section

Falsely claimed to be Telstra technicians, claiming to shut down my internet because of malicious code. Wanted me to connect my computer to their website so they could remotely get rid of the code.