"Peters himself will soon be retired and will require a care worker to help him get dressed and go for a walk. He'll discover that such facilities can't function without migrant workers," he added.
Peters wasn't having it, rolling up his sleeves and hitting back with his own tweet.
"I've spent much of my career respecting and working for retirees," said Peters.
"You seem to want to euthanise them. As for your nasty comments about my physical - I reckon you'd last 10 seconds in the ring with me."
In a separate tweet, Peters clarified that, despite his claim, he is not into physical violence but any hypothetical encounter between the pair would be brief.
"There'd be three hits - you hitting me, me hitting you, and the ambulance hitting 100," Peters wrote.
The argument spiralled from an Act Party release this morning, in which Seymour criticised NZ First immigration policy for undercutting the need many industries have for migrant workers, using healthcare as an example.
"The ongoing growth of New Zealand businesses depend on being able to get workers, from engineering to aged-care facilities," Seymour said in the release.
"Nobody has to worry because NZ First never keeps a significant promise. They promised to slash immigration last time, and nothing happened," he added.
Seymour responded to Winston Peters' attack, saying he had hit Winston "where it hurts".
Not to be outdone, NZ First's official Twitter account responded with an illustration showing the two men in the ring.
The Act Party is the most recent party to earn Peters' ire, after he sent shots across the bow of both the National and Labour parties at his party's annual conference.
He lambasted National for not costing the four-lane expressway from Whangārei to Tauranga, saying it was "breathtaking stuff".
"Who do they think they're fooling?"
And, despite the fact that National has ruled out forming a coalition with NZ First after the election, Peters said the party's new leader, Judith Collins, will be picking up the phone to call him on election night nonetheless.
"You've got to bite the bullet sometimes – if you don't want to do that, then get out of this business."
And Peters was pulling no punches when it came to his coalition partners – Labour – either.
"None of my ministers have been screwing up – none of my ministers have had to be sacked," he told the Nation.
This appears to be a pointed reference to Health Minister David Clark, who resigned a few weeks ago after months of pressure.
He also took aim at the Green Party, saying NZ First "have opposed woke pixie dust".
"Whilst the rest have been politically correct, we've set out to correct politics. We've been an accelerator for good ideas and a handbrake for bad ones."
Peters said this election campaign would show how important NZ First was to the country.
He took aim at the lack of experience of both Labour and the Greens.
Peters claimed that "questions over competency or controversy have never been an issue for us".
