Waikato Regional Council says it appears most of the contents of a tanker carrying almost 25,000 litres of dairy product that crashed in the Awakino Gorge have not gone into the nearby river.
The truck and trailer unit went over a bank on State Highway 3 about 5.30am near Awakino River in Waikato on Friday.
Emergency services say the driver was trapped for about an hour before being flown to Taranaki Hospital where he was being treated for serious chest injuries.
The truck was carrying evaporated whey permeate, a by-product from cheese-making.
Council emergency response manager Rob Dragton says the sugar-rich material starves water of oxygen as it dissolves, which makes it toxic to fish and other aquatic life.
Mr Dragton says initially it was thought about 20,000 litres of the sticky substance got into the water. However, he says only about 2000 litres from the trailer unit leaked out, with the rest either remaining in the tanker or leaking onto the ground where it was cleaned up.
The whey permeate was being pumped out into another tanker.
State Highway 3 - the main route between Te Kuiti and New Plymouth - will be closed intermittently on Friday as a crane removes the wreck in three sections.
The Transport Agency says each closure could last up to 45 minutes.
An alternative route via State Highway 4 is available but would add significantly to travelling times.