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Ardern visited her grandmother in Waikato Hospital on Sunday after a campaign rally in Hamilton.
It is understood her grandmother died last night after having a stroke last week.
A funeral is expected to take place later this week.
Just before she found out, Ardern had spoken about attempts to send her grandfather home from Waikato Hospital at 11.30pm at night because the hospital was full - saying it showed the need for more investment in health.
Ardern is campaigning in Wellington today and focusing on health.
Labour released a "government report card" today which they claim shows the health system is underfunded.
Ardern has previously credited her family with sparking her interest in politics. She has a framed copy of the newspaper front page that shows her grandmother meeting Labour prime minister Norman Kirk.