While debate rages over Australia's border security, there's growing evidence that the greatest threat to Australia's national security potentially comes from foreign computer hackers. Few in government or business will admit the full extent of the break-ins, with one expert calling it a "dirty little secret".

Next on Four Corners reporter Andrew Fowler reveals that hackers, working from locations overseas, have targeted key Federal Government departments and major corporations in Australia. Their intention is to steal national security secrets and vital business information.

In one case, an Australian company that supplies secret communications equipment used by military across the globe had its computer network hacked. It appears the hackers accessed the system holding vital design information involving a military radio system. The break-in meant secure communications used by Australia's allies could be compromised.

Speaking with security specialists and insiders, Four Corners also details a number of specific high level break-ins involving Government departments. In each case it explains how the security system might have been breached.

A deafening silence surrounds this issue. Companies won't speak about the break-ins because they fear it will alarm clients and shareholders. Governments refuse to speak up because inevitably they will be asked, who is doing this? The answer is uncomfortable.

A number of people, including former government advisors in cyber security, claim the digital trail leads to China. Although it's unclear if the hackers are working for the Chinese Government, those same experts believe that any company doing significant business in China must assume it will be the target of corporate espionage.