Williams said the previous spike was due to dairy prices, whereas recent gains were across forestry, horticulture and meat as well, meaning it was "more widely spread across sectors, regions and individual businesses".
The index shows dairy prices rose 3.8 per cent last month and were more than 50 per cent higher than a year earlier, with higher prices across most milk powders and milkfats.
"Global prices were supported by lower peak milk flows in Europe's major producing countries such as Germany and France, due to cold spring conditions," Williams said. "Continued demand from China and broader Asia also continued to boost prices."
Meat prices rose 2.8 per cent, with beef prices up 2 per cent and lamb prices up 2.8 per cent, although wool prices remained "lacklustre" with a 0.3 per cent monthly decline taking the annual drop to 34 per cent.
Aluminium prices slipped 0.4 per cent in May having gained 23 per cent over the past year, while seafood prices edged up 0.4 per cent.
Wood pulp prices increased a monthly 2.6 per cent and log prices gained 0.6 per cent.