By Charlotte Greenfield and Ana Nicolaci da Costa
WELLINGTON (Reuters) - The final opinion poll before New Zealand's election on Saturday showed the ruling National Party's lead over the opposition Labour Party had slipped to 8.5 points, meaning minor parties could well decide who forms the next government.
The Newshub-Reid survey released on Thursday showed support for Prime Minister Bill English's National Party had edged down 1.5 points to 45.8 percent, but support for Labour also fell, dropping 0.5 points to 37.3 percent.
The National Party has governed for the past nine years, and its re-election would hearten investors who favour continuity and like National's vow to keep the small trading economy open to foreign investment.
Regardless of the polls, which have shown sharp divergences, the nationalist New Zealand First Party could well emerge as the kingmaker in the next parliament.
Minor parties often have an outsize role in New Zealand's proportional representation system, in which a party, or combination of parties, needs 61 of Parliament's 120 members - usually about 48 percent of the vote - to form a government.
The Newshub-Reid survey showed support for New Zealand First hitting 7.1 percent, as did support for the Green Party, which is a likely coalition partner for Labour.
The tight race, with Labour and the Green Party virtually neck-and-neck with National, meant the government might not be known for weeks.
"Despite all of the volatility, despite all the twists and turns of the campaign, it has come back to that simple fact ... New Zealand First appears to be that crucial kingmaker," said Bryce Edwards, analyst at Wellington-based Critical Politics.
Outspoken New Zealand First leader Winston Peters has said he will not confirm which bloc of parties he'll support until October 12 when the results become official.
The election was so hotly-contested that the deciding votes might be those of overseas New Zealanders, whose ballots would not be released until Oct. 7.
These votes, as well as those from New Zealanders who voted outside of their home area, accounted for around 12 percent of total ballots cast in the 2014 election, according to the Election Commission.
Labour leader Jacinda Ardern said on Wednesday that citizens living in Australia, home to New Zealand's largest diaspora, were "very important" to the election.
"Most New Zealanders maintain a strong interest in the direction of the country, particularly when it comes to things like how we're grappling with international issues, trade issues, environmental issues - so please have your say," Ardern told the Australian Broadcasting Corporation.
The New Zealand dollar - the word's 11th most traded currency in 2016 - briefly firmed to $0.7324 from $0.7312 after Thursday's opinion poll, but quickly gave up most of those gains and sank to $0.7305.
(Reporting by Charlotte Greenfield and Ana Nicolaci da Costa; Editing by Simon Cameron-Moore)