The second man remained on board and with his torch tried to show rescuers his ever-changing position. He also described to police over his cellphone key bridges as he passed under them.
"The guy left with the boat was trying to figure out where he was," said the spokeswoman.
"He had a torch and was flashing it and trying to figure out what bridges they were between."
Eventually the foundering craft was found near Mercer, 52km from Ngaruawahia, and police and fire boats rescued the man.
A police spokesman said it was not clear why the pair were on the river in the first place but both men were safe.