Mr Mason failed to keep hold of the raft but was seen on a bank shortly after, Sergeant Denis Foster told the inquest.
"For some reason, Mr Mason jumped back into the water in an apparent effort to get back to the raft. [He] failed to rejoin the raft, passing completely underneath it."
The rest of the group, who last saw Mr Mason positioning himself on his back with arms folded across his chest to negotiate the rapids, raised the alarm when they were unable to find him.
His body was found 10km downstream the next day.
Mr Foster said Mr Mason, who had recently been introduced to hunting and rafting by the Ardens, had thrown himself into the sport "with relish".
"He was aware of the risks involved in rafting the Motu but took time off work so that he could enjoy his newfound passion."
Asked by Coroner Wallace Bain whether the river conditions were overly hazardous at the time, Mr Foster said experienced rafters had negotiated the river at faster rates.
But both agreed Mr Mason's decision to re-enter the river after he had made it to shore was unwise and unfortunate.
Dr Bain concluded with a message to rafters in raging rivers: "If you get chucked out and end up on the bank, you should stay there."