Greens' co-leader Marama Davidson has labelled ACT's no-show at Rātana today a display of "absolute ignorance" and a dishonour to the Māori world.
Seymour has never attended Rātana festivities, describing it as a religious event, but will be at Waitangi next month.
Opposition MPs were welcomed onto the pā this morning, followed by a contingent from National and New Zealand First.
ACT's absence was an abdication of responsibility, Davidson said.
"It dismisses the mana and the importance of Ratana, of Wiremu Pōtiki Ratana, and te ao Māori and their political voice."
She accused ACT leader David Seymour of playing to fear, ignorance, and division by "appealing to the worst of us" through his efforts to redefine Treaty principles.
Davidson challenged Prime Minister Christopher Luxon to use the day to listen to Māori.
"He shouldn't say much at all. He should listen, he should receive and then he should ensure his government continues to uphold ... Te Tiriti o Waitangi.
"Stop using Māori as your racist drawcard."
Senior Labour MP Kelvin Davis also took a serve at Seymour when speaking to reporters at Rātana.
"It really annoys me when David Seymour says things like 'we've never had an adult conversation about the principles and the Treaty of Waitangi'. I don't know where he's been for the last 184 years.
"David Seymour is a Johnny Come Lately who has never engaged in the Māori world, on those issues that are important to Māori. And all of a sudden he thinks that he knows it all. It's just ignorance playing out."
Davis said he was sure the government representatives would get a respectful reception at Rātana.
"This is a peaceful, respectful hui, it always has been, and I don't expect that to change. But I think people will still take the opportunity to express how they're feeling."
Several of the speakers on the hau kāinga side at Rātana referred to the "three-headed taniwha" government, and noted Seymour's absence, despite the presence of leaders from all the other parties in Parliament.
Speaking from the Beehive in Wellington, Seymour defended his non-attendance.
"Why would you be there? There are thousands of events up and down New Zealand every day, you needn't go to all of them," he said.
"My understanding is a guy came out as a prophet of his own religious movement in the 1870s. And politicians feel a strange obligation to be there every year. I've never felt that."
Seymour said he would instead be up at Waitangi for "the country's most important national holiday".
"When people say that somehow Rātana is the sine qua non (essential ingredient) of political discussion in New Zealand, they might be jumping a few sharks.
"We have an annual event to commemorate the Treaty of Waitangi, our founding document, and I'm looking forward to being up there in a week or two."