Booming Indigenous Maori "haka" chants rang out across New Zealand's capital on Tuesday, as tens of thousands rallied against a conservative push to redefine the nation's founding treaty.

More than 35,000 demonstrators poured into the city of Wellington, police said, shutting down busy streets as their spirited procession inched its way towards parliament.

Bare-chested men draped in traditional feather cloaks were joined by horse riders waving the red, white and black Maori flag.

Children marched alongside adults bearing distinctive full-face Maori "moko" tattoos and clutching ceremonial wooden weapons.

"The vibe is beautiful. People of all walks are here to support. It is peaceful and respectful," participant Nick Stewart told reporters.

Protests have been swelling throughout New Zealand after a minor party in the conservative coalition government drafted a bill to redefine the 1840 Treaty of Waitangi.

Although the bill has almost no chance of passing, its mere introduction has triggered one of New Zealand's largest protests in decades.

Many critics -- including some of New Zealand's most respected lawyers -- see it as an attempt to strip long-agreed rights from the country's 900,000 strong Maori population.

"We stand here gathered in our tens of thousands on the steps of parliament to protest this bill for our mokopuna," said Stewart, using a Maori word for future generations.

Crowds gathered on the lawns outside New Zealand's beehive-shaped parliament building, playing reggae music and giving speeches that urged the government to come out and face them.

"It's not the best way to have a conversation. We will not accept unilateral change to a treaty that involves two parties," said Ngira Simmonds, a key advisor to New Zealand's Maori queen.

Many demonstrators arrived in Wellington after a nine-day "hikoi" -- or protest march -- that began hundreds of kilometres away at New Zealand's northern tip. (AFP)