"Just because they announced it doesn't mean farmers will get it. The market is still pretty volatile out there and farmers should continue to be mindful of their budget," he said.
"Aside from loans with commercial banks, farmers also have Fonterra loan to pay back and anything above $6 goes into that repayment and I don't know anyone who didn't take that loan to keep going in the last two years."
But Mr Cullen said the final forecast price was not likely to fall too much from current predictions, saying it was highly likely to be above $6.
"The writing's on the wall for this to be a good season pay wise but we still have to err on the side of caution."
He said dairy farmers in Northland endured a wet start to the season which hampered grass growth and damaged pasture.
Fonterra chairman John Wilson said the increased forecast price would be welcome news to farmers.
"We are seeing growing confidence on-farm across the country and, with global demand for dairy strengthening, the signs are for a good start to the season for our farmers and their rural communities although following a challenging period of very wet conditions for some of our farmers," he said.
Company chief executive Theo Spierings said an increasing number of consumers in Fonterra's global markets preferred dairy for their everyday source of nutrition which was converting into strong demand, particularly in consumer and food service products.
The forecast price hit $6/kgMS in November last year .
