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The Flower St building houses its television head office and studios.
It is understood staff were told just six minutes before the announcement was made public.
Staff fear that if a buyer cannot be found quickly the network could be closed by Christmas this year, a source said.
TV presenters had been called to one-on-meetings - though some of the network's biggest names are currently away. Newshub Live at 6 anchor Mike McRoberts is in Japan for the Rugby World Cup and Samantha Hayes is on maternity leave.
A newsroom meeting was planned for this morning, with staff not at work today asked to attend, the source said.
The sell-off follows a concerted effort by the broadcaster to complain about its disadvantage taking on government-owned TVNZ. Just this week MediaWorks axed or reduced several comedy shows and Married at First Sight NZ.
One casualty was comedian Guy Williams' show NZ Today.
Seven Days - one of MediaWorks' most successful shows - was cut from 32 episodes to 12 this week.
MediaWorks concedes defeat in 'challenging' TV environment
MediaWorks, which is owned by US hedge fund Oaktree Capital, will continue to operate its radio and billboard advertising businesses.
The company posted a $5.5 million loss for the 2018 year.
MediaWorks chairman Jack Matthews said the company was committed to growing its business in New Zealand "while recognising that free-to-air television operates in a challenging environment".
"We are in a commercial environment and have to face commercial realities," he said.
"The market that free-to-air television operates in is tough and has been exacerbated this year. This is reflected in the performance of all free-to-air television operators in New Zealand, not just us."
MediaWorks planned to sell its Flower St building, with a possible leaseback option "for a buyer to continue to operate television from that location".
The company has now hired an adviser to "identify a list of potential purchasers and commence initial discussions".
The sell-off comes just days after The Project host Jesse Mulligan warned Three was facing serious challenges.
"What's the problem? Well, Three has a big competitor," Mulligan said on live TV.
"You will have heard of it — TVNZ. They do everything they can to take viewers off Three, and that's what they should do. But at the moment it's not a level playing field."
Mulligan also warned that MediaWorks was "out of costs to cut".
"It's not unrealistic or an exaggeration to say the next move could be for MediaWorks to close down our entire station."
Chief news officer Hal Crawford publicly complained in August that television in New Zealand was "broken".
MediaWorks chief executive Michael Anderson said today: "We are very pleased with the progress that Three has made over the past five years. This year we have had record ratings and revenue share highs and we are very proud of our stellar local content.
"The role Three plays in New Zealand society is significant, from Newshub through to investment in local comedy and drama. We believe MediaWorks TV is now in a place where it can be separated from the radio and outdoor business to be operated under a new owner in a more sustainable fashion - and, ultimately, for profit."