The Government must negotiate a new trade deal with Britain after Brexit, an expert says.

Oliver Hartwich, executive director of business lobby group The New Zealand Initiative (formerly the Business Roundtable) executive director, is encouraging swift action to further this country's economic and political interests.

"For New Zealand, this is a great opportunity. As a small, free-trading nation, New Zealand has expertise in negotiating trade deals such as the free trade agreements with China, Hong Kong and South Korea.

"The New Zealand Government should make this expertise available to the UK government and work constructively towards a New Zealand-United Kingdom free trade agreement. If such a deal includes Australia, it would be even better," he said.

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Boris Johnson, the leading candidate to replace outgoing Prime Minister David Cameron, has previously called for a strengthening of Britain's ties with Australia and New Zealand, Hartwich noted.

"In one of his Daily Telegraph columns in 2013, Boris Johnson called for a Free Labour Mobility Zone between the UK and Australia and New Zealand. Mr Johnson is obviously interested in reviving the ties that Britain's entry into the Common Market in 1973 severed. Should he become Britain's next Prime Minister, the New Zealand government should cooperate closely with Mr Johnson in deepening the relationship between the two countries," Hartwich said.