Medibank hackers may be in Russia, as well as in "other countries", say police, as Optus goes silent on ransom claim.

Almost a month after Optus suffered one of the worst data breaches in Australian history it continues to run a “completely insecure” website, says a top cybersecurity expert.

Telco giant Optus runs a “world-class” global cyber security business but is refusing to say whether it was used to protect its Australian customers, the group’s biggest source of profit.