"In this environment, Shortland Street is the backbone of TV2," said John Barnett, owner of South Pacific Pictures, maker of the show.
Barnett told The Diary TVNZ would only accept the production on a yearly basis in the beginning.
"At the end of the first year, we renewed it for another year. No one would have thought 20 years later we'd be standing here."
No expense was spared at the birthday celebrations.
The party carried on until midnight with musical performances by Jupiter Project, Late Nyte Hype, Dane Rumble and Zowie.
TVNZ executives Andrew Shaw and Jeff Latch, who are at industry screenings in California, filmed a pre-recorded message wearing Shortland Street scrubs. They acknowledged the importance of the show on the New Zealand television landscape.
Shorty St has made SPP a very powerful player in the industry. They are the largest film and television company in New Zealand. The Listener named Barnett "the most powerful studio boss in television".
Barnett owns SPP with UK conglomerate All3Media, the largest independent television producer in Britain. They have a stable of hit shows, including Hollyoaks, Midsomer Murders, The Only Way is Essex and Ramsay's Kitchen Nightmares.
All3Media is owned by Permira, a €20 billion ($33 billion) British private equity firm. It tried to offload the media company last year, but could not reach an asking price.
Shorty St is backed by big, big coffers from offshore venture capitalists.
Those dollars did not contribute to last night's party. It was a local network saying thanks to a local production for 20 wonderful years.