The vote marks a new era in Finland, which for decades has elected presidents to foster diplomacy, in particular with neighbouring Russia, and opted not to join military alliances so it could soothe tensions between Moscow and NATO.

But Finns changed their minds about playing that role after Russia's invasion of Ukraine in 2022, in a rapid u-turn that led to Finland joining NATO in April last year.

Now under the Western alliance's security umbrella, the new president will replace Sauli Niinisto, who is retiring after two six-year terms in which he earned the nickname "the Putin Whisperer" for his previous close ties with the Russian leader.

Niinisto's successor will have a central role in defining Finland's NATO policies, while leading on overall foreign and security policy in close cooperation with the government and acting as a commander-in-chief of the armed forces.

Nearly 46 per cent of those entitled to vote cast their ballots in advance of Sunday, official data showed, with the first results from advance votes due to be published shortly after polls close at 0500 AEDT Monday. The winner is expected to be known by around 0800 AEDT.

Stubb, a former prime minister, won the election's first round on January 28 with 27.2 per cent of the ballots ahead of liberal Green Party member Pekka Haavisto on 25.8 per cent. He has also led Haavisto in surveys, most recently by six to eight percentage points.

Both candidates are pro-European and strong supporters of Ukraine who have taken a tough stance towards Russia in their campaigns.

In an interview with Reuters last month, Stubb said there would be no Russian pillar in Finland's foreign policy for now: "Politically, there will be no relations with the president of Russia or with the Russian political leadership until they stop the war in Ukraine."

Stubb is in favour of deep NATO co-operation, such as allowing the transport of nuclear weapons through Finnish soil and placing some NATO troops permanently in Finland. He does not support storing nuclear weapons in Finland, however.

Haavisto, a former foreign minister who has also served as a UN peace negotiator and is known as a human rights defender, has called for a more cautious approach.

Haavisto wants to maintain Finland's ban on nuclear weapons on its soil and considers a permanent NATO troop deployment unnecessary for the current security situation.

Australian Associated Press