The BNZ's warning about a new economic risk that could blow in and hit our trade balance hard.

It's a possible El Nino weather pattern.

The odds of that happening are being put at around 70 percent at the moment.

Forecasters say it's usually associated with dry conditions in the east and wind from the south west -and it can negatively affect primary production.

It can also restrict hydro electricity production.

The risk is one reason by the BNZ's economy watchers haven't lifted milk production forecasts despite a strong start to the season.