Mr Barnett said last night the sale could help New Zealand programming make its way on to the world stage.
Mass media and entertainment company Discovery Communications produces the Discovery Channel and a range of other non-fiction products and had a revenue of $6.41 billion last year.
Communications company Liberty Global is the largest international cable television company in the world and is chaired by America's largest landowner, billionaire John Malone.
Mr Barnett said the purchase was good news because it provided a new market for their shows and would not affect the day-to-day operations of South Pacific Pictures.
The new joint venture of Liberty Global and Discovery Communications meant there was a ready-made market for sale of rights to cable TV channels for sale of shows for "second runs". These are shows that had already been sold to broadcasters for their first run.
Despite remaining chairman on South Pacific Pictures Mr Barnett said he did not stand to get a big payment from the sale.
Liberty owns more than 200 channels and 14 territories and its holdings include UK firm Virgin Media. All3Media productions include Midsomer Murders, Shameless, Skins and Undercover Boss.
All3Media is the largest independent production group in the UK with headquarters in London and also has strongholds in Germany, the Netherlands, Australia and New Zealand.
All3Media chairman and co-founder Steve Morrison said: "This new partnership will enable All3Media to expand further through multiple avenues of growth around the world."
The company will continue to operate under its own name with its own executive management team, and its existing creative operating model as a standalone, independently managed entity.
The transaction is expected to close in the third quarter of this year.
The sale was big international news in the telecommunications and television sector, with media coverage by the LA Times, the Wall Street Journal and Hollywood Reporter over the weekend.
