Labour saw a major surge after the July appointment of the 37-year-old leader but has fallen back slightly since.
The vote is expected to result in a coalition government with one of the minor parties as kingmaker.
Major campaign issues have been the economy, infrastructure, healthcare and housing.
The election had initially promised limited surprise until Ms Ardern took over the Labour leadership, leading to a sudden surge and bringing her party back into contention.
Ms Ardern has targeted young New Zealanders with policies on education subsidies, housing and the environment.
The strong Labour support, dubbed "Jacinda-mania", which brought the party neck and neck with the National Party dropped back in the last days of the campaign.
Her main rival is the current centre-right Prime Minister, Bill English, who has promised voters experience and reliable economic management.
Mr English became prime minister in 2016 when his predecessor John Key unexpectedly resigned.
Given New Zealand's proportional representation system, smaller parties like the Greens or New Zealand First are expected to win seats. Neither of the two big parties are expected to win an outright majority.
This means the next government is likely to be a coalition with one of the small parties.
Voting opened at 9:00 local time (21:00 GMT Friday) and closes at 19:00 local time (7:00 GMT).
The first preliminary results are expected shortly afterwards while official results will be announced on 7 October.
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