The witness tackled the offender and attempted to protect the injured man, trading blows with the man before managing to wrestle the bat from him.
The offender ran off with another man but police patrols were unable to locate them.
The victim was taken to Nelson Hospital with a severe cut to his head and a suspected broken arm.
Mr Kaveney said it appeared the victim had earlier spoken to the offender about his behaviour at the hotel.
The offender then left the hotel and waited for his victim to leave before launching the attack.
"This is a premeditated and unprovoked attack on an innocent person, which has resulted in that person being seriously injured,'' Mr Kaveney said.
The offender was described as a stockily built European man in his mid- to late-20s who was unshaven and had short to shaved hair.
The man he ran off with was a European in his early 20s, also with short to shaved hair.