She has also announced the 2023 election will be on October 14.
Ardern said she is entering her sixth year in office and it has been a challenging role.
You cannot and should not do the role unless you have a full tank, she says. She said she hoped over summer she would be able to prepare for another term, but she has not been able to do that.
Newshub reports the Prime Minister said her term has seen a major biosecuity incursion, a terror attack, a natural disaster, a global pandemic and an economic crisis.
"I know what this job takes and I know I no longer have enough in the tank to do it justice. It's that simple," Ardern said.