At first New Zealand traded with Australia but from the 1870's Great Britain became New Zealand's main trading partner. New Zealand mostly traded in food and agricultural products, and its dependency on the one market meant it was sometimes known as Britain's Farm.
When Britain joined the European Economic Community (EEC) in the 1960's New Zealand became concerned that it would be harder to compete for British trade, and so needed to diversify its trading partners.
By the early 2000s New Zealand had many trading partners, and had setup free trade agreements with countries like Singapore, Thailand, and China.