Organiser of the "Keep Our Assets" protest, Grey Power president Jo Millar, said she was "absolutely over the moon" to see so many people participating, particularly children.
"That's who we are doing it for," she said.
The organisation initiated the protest, attended by a range of political parties and unions, because "we are wise old owls and have learnt by our previous mistakes".
"We do not want you to have to put up with the mistakes that are going to be made if these people sell our assets," she told the crowd.
About 75,000 people had signed a petition calling for a citizens-initiated referendum on the issue, which required 300,000 signatures to trigger.
"They say referendums are not binding. Well, this one is going to be if he [John Key] wants to survive.
"We don't want to sell the assets. We are the majority," she said.
Protesters marched along George St to the Octagon carrying banners reading "Don't let this donkey sell our assets"; "Touch my assets, your ass is on the line"; and "People's power not privatised power".
Chants rang out, including "John Key, you've got mail, Aotearoa's not for sale" and "Stop the thieves in time, asset sales is a crime".
Green Party co-leader Metiria Turei praised the crowd for attending as it was "you that makes the difference", she said.
"You are the future for our children and it's by standing up and making a stand now . . . that you will make our future for our kids even better."
Labour MP David Parker said "people power is alive in New Zealand", highlighting the turnaround on increases to school class sizes, and told those gathered they could make a difference because the assets were "not [the Government's] to sell".
Private power companies charged $265 a year more than their state-owned equivalents, so selling state-owned power companies would lead to power price increases, he said.
"This is just economic lunacy. It increases inequality and increases the Government deficit by $100 million per annum ... We've got to fight it."
New Zealand Nurses Organisation representative Glenda Alexander agreed the sales did not make economic sense as "mum and dad investors" the Government said would buy shares in the companies were the same people struggling to pay their power bills each month.
"What we are concerned about is we haven't been asked if we want to sell our assets or not."
