The company had previously expected around $530 million.
Mercury says the potential rise in earnings is due to an increase in its full year forecast hydro generation due to ongoing wet weather in the Taupo area.
Annual hydro generation is now expected to be around 700 gigawatt hours (GWh) above average for the 2018 financial year.
The company says guidance may change and remains subject to any material events.
Shares in Mercury NZ (ASX:MCY) closed down 3.94% to $3.17 yesterday.