Most people spend more than 25 per cent of their time online at work on personal activities.
And 80 per cent of emails sent by volume in the workplace are personal.
Bosses often have no way of tracking internet activity or policies to define what staff can andcannot do.
Paul Hortop, who reviews company network security for consultancy Voco, said the most common websites visited by personal web surfers were online trading sites, instant messaging/chat services and peer-to-peer sharing sites (allowing movie, music and software sharing).
It was a challenge determining what was appropriate internet use, Mr Hortop said. For example, monitoring bidding on a ski jacket or trying to find love was private business but social networking can be an asset to a company and a private distraction.
"Is it more time-efficient to let staff do their banking online than having them leave the office for half an hour?" he asked.
Mr Hortop, who compiled the information from his own investigations and from a survey by online advertising provider Burst Media, said management were often unaware what was happening on networks under their control.
He had discovered several employees had illegally downloaded the supernatural thriller Hellboy: The Golden Army on corporate networks ahead of its official release date.
"I'd be inclined to say Hellboy fits into that geek community where people are technology-literate and using peer-to-peer file sharing.
"Often the people abusing resources will be more technology-literate than the people responsible for the security of the network. CIOs and CEOs are often a little distant from the technologies they're responsible for."
The company, not the employee, would face the legal implications if investigators tracked who was downloading movies.
But employers might get more tough after the Copyright (New Technologies) Amendment Act is brought in to force this year. The act says internet service providers should implement a policy providing for termination of customers who have not respected copyrighted material.
