Speaking before the ceremony, Mr Harawira said he did not find it difficult to swear an oath he did not believe in, comparing the task to having to wear a suit at Parliament.
"You change your lifestyle to suit the things that you are doing, and if it means that the people of Mana want to see me take my place in Parliament then that's what I'll do,'' he said.
Mr Harawira's initial refusal to swear the legal oath renewed debate about whether the oath needed to be updated, and the Maori Party today announced that it would be introducing a new bill to address the issue.
The Oaths and Declaration Bill would give politicians, police officers, judiciary members and public office-holders an option to uphold the Treaty of Waitangi when they swore their oath of allegiance.
"The Treaty of Waitangi is our founding document and people who hold public office should not be deprived of giving it recognition,'' Maori Party law and justice spokeswoman Rahui Katene said.
Mr Harawira said most New Zealanders thought it was time to change the oath and he would work with the Maori Party to try to get it amended.
"This is about Aotearoa, eh. This isn't about the Queen, it's about us.''
Green Party co-leader Russel Norman said the oath was a bit antiquated and there should be a cross-party discussion about how it could be updated.
However, the bill will struggle to get off the ground, with Prime Minister John Key having already said he was happy with the current oath and Labour appearing unlikely to get behind it.
"I can't see people wanting to sign allegiance to the Treaty of Waitangi, I mean most New Zealanders again recognise that the treaty was the founding document of modern day New Zealand but we swear allegiance to the Queen and we swear allegiance to the constitutional arrangements that New Zealand has,'' Mr Key said.
"If that position changes and New Zealand becomes a republic then we might want to go and re-look at our oath, but outside of that I can't see any reason why we would want to spend any time looking at changing it.''
Labour leader Phil Goff said he could see merit in changing the oath.
"But if that was to happen, I'd prefer to see it go beyond simply an oath of allegiance to the Head of State to include swearing commitment and loyalty to New Zealand and its people.''
- NZPA