Her daughter Neve tested positive on Wednesday, she added in the post. Her partner Clarke Gayford tested positive on Sunday.
"Despite best efforts, unfortunately I've joined the rest of my family and tested positive for Covid-19," Ardern wrote on social media.
Ardern has been symptomatic since Friday night, according to a statement, and has "moderate" symptoms. She returned a weak positive Friday night and a clear positive this morning on a RAT test.
Ardern will be required to isolate until the morning of Saturday 21 May.