This has triggered a byelection in his Botany seat, but Ross said he will be standing as an independent.
He said he is confident he can win the seat as an independent candidate.
"I'm confident I can run on a track record of 15 years."
Ardern said Botany has traditionally been a strong National seat.
"This, however, is going to be an interesting byelection to say the least."
She said having Ross running as an independent will give the election a "different dynamic".
"But no doubt, this is going to be a very different byelection than what we're used to in New Zealand."
Ardern said she will be having conversations with the Labour Party about "what they would like to do in this race".
Green's Co-Leader James Shaw said whether or not the party stands a candidate is up to its executive.
"We haven't even begun to examine that," he said, but adds that it is being considered.
"We will have to get to that bridge when we get to it."
NZ First Leader Winston Peters said his party has not made a decision about standing a candidate yet.
"We always make those decisions on the basis of the caucus – it's party discussions which we haven't had.
When pressed, he said: "We don't make decisions off the top of our head, we're famous for consultation."
ACT Leader David Seymour said the party would probably stand a candidate, but it was up to the board as to who that will be.
In last year's election, Ross won the electorate by a significant margin – winning 21,400 votes to the Labour candidate's 8500.
It was a similar result in 2014.