Police say about 5000 joined a hikoi through the central Wellington on Friday afternoon while 1500 assembled on the steps of Parliament to protest against partial privatisation of state assets.
The Government plans to sell up to 49% of the state-owned power companies and reduce the Crown's holding in Air New Zealand.
Several central Wellington streets were closed as the group marched through the city from Te Papa Museum to Parliament.
The protesters were at the end of a hikoi which began at Cape Reinga in late April and were greeted by members of opposition parties including the Maori party which is opposed to the sales.
Radio New Zealand's reporter says groups within the demonstration are also protesting against the proposed Trans-Pacific Partnership, against fracking and in support of saving Maui's dolphins.
The Prime Minister, however, has dismissed the protest.
Mr Key, who was in Auckland at the time of the protest, says he does not intend to step back from the sales.
"Well over a million New Zealanders voted for National in the full knowledge that we were going to undertake the mixed ownership model. So a few thousand people walking down the street in Wellington isn't going to change my mind."