Ms Ardern and partner Clarke Gayford arrived at St David's Cooperating Parish soon after 11 and walked into the service hand in hand.
Her parents Laurell and Ross Ardern were also at the funeral, which was well attended by the local community.
Ms Ardern will return to the campaign trail straight afterward for the last day of campaigning. She will visit malls in Auckland before events in her Mt Albert electorate.'
Ms Ardern learned her maternal grandmother, Margaret Bottomley, was unwell while she was campaigning on the West Coast last week.
She visited her grandmother in hospital last Sunday.