There are concerns for a pair of rafters who've been missing overnight, after an adventure on the Hutt River went wrong.

They were part of a group of eight who got into trouble in the Akatarawa Valley.

Six of them were winched out by helicopter, but darkness and the dense forest prevented the other two being found.

Life Flight crewman Julian Burn says the six they rescued were obviously very worried.

"You are going to be concerned if there is people out overnight and you don't know the condition that they are in, whether they are injured or not."

He says after transporting the six rafters to safety, the helicopter crew doubled back to look for the others with no luck.

"Unfortunately for us, some of those walking tracks are quite heavily forested with dense canopies, night time in the helicopter it makes it pretty difficult to be able to see under that obviously."

Burn says teams searching on foot should have a better chance of finding the pair.

It's not known if either of the missing pair was injured.