Wellingtonian Matthew Plummer has sent in his "Leave" postal vote - one of thousands of Kiwis who will help decide the Brexit referendum.

Anyone with a British passport who lives here is eligible to vote in the referendum, provided they have registered to vote in the past 15 years.

There are about 255,000 British nationals in New Zealand, although not all will hold passports or have registered to post ballot slips back to the UK.

According to polling there is little separating the "Remain" and "Leave" camps - leading the BBC and other media to note that expats in New Zealand and other countries like Australia could swing the referendum.

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Mr Plummer, 38, describes his nationality as "half and half", having attended school in Britain and university in New Zealand.

He said the referendum was the biggest question voters faced in a generation.

What he saw as the European Commission's lack of accountability to British voters was one reason behind his decision to tick Leave.

Prime Minister John Key said yesterday that a Brexit would have "some impact" on New Zealand, but our economy was far less reliant on Britain than in the past.

"Compared to where we were in the 1960s and 1970s, Britain is just not a really major force these days from our point of view."

A sharp fall in the pound could affect tourism and other areas, Mr Key said.

The referendum has been a talking point at the Best of British food shop in Hamilton.

Owner Lee Webster, who has been in New Zealand about 20 years, said most shoppers wanted Britain to leave the EU, a view he shared.