Export of coffee across the globe has witnessed a big leap in the crop year that began in October.

According to International Coffee Organisation, global coffee exports rose over 16 per cent during the crop year due to an increase in shipments of robusta varieties from Vietnam.

Exports rose to 65.87 million bags in the eight months through May, compared with 56.54 million a year earlier.

Robusta shipments surged 28 per cent to 23.9 million bags (one bag 60 kg), and exports of arabica beans rose 11 per cent to 41.97 million bags.

The growth in robusta exports has been limited by the poor quality of some beans from Vietnam, the world's largest grower of that type of coffee, the ICO said.

Rising demand for robusta has sent prices in London up 38 per cent in the past year, reaching a nine-year high last month.

The supply of good quality robustas is below the level of demand, ICO said. Brazil, the world's biggest producer and exporter, probably will harvest 32.1 million bags in the harvest that runs through October, down 24 per cent from 42.5 million bags the previous year.

Output is down because trees are in the lower-yielding half of a two-year production cycle. The decline in Brazil will contribute to a 7.9% drop in global coffee output to 112 million bags.