A disappointing 10th anniversary for Fonterra's Global Dairy Trade auction.

Overall prices have slumped five-percent at the latest overnight auction -- the biggest drop of the year.

And it's even worse for the crucial whole milk powder price, down 7.3 per cent.

JMI Wealth director Andrew Kelleher told Mike Hosking it's not something anyone expected.

"The dairy index was under a bit of pressure in late 2017 but it had recovered in late 2017 which was a surprise."

LISTEN TO ANDREW KELLEHER TALK WITH MIKE HOSKING ABOVE