"Bruce has spent the weekend convalescing with family and friends but he needs further hospital treatment for his facial and head injuries as part of his longer term rehabilitation," Central police district commander, Superintendent Russell Gibson, said.
Mr Mellor, his family and friends had appreciated the huge number of messages he had received -- many from strangers.
"These generous and supportive messages have boosted Bruce and all of us," Mr Gibson said.
Mr Mellor is a "humble and private" man.
"He's overwhelmed by the hundreds of cards and messages of support he's received and by the lovely flowers which have brightened his days and those of people looking after him," he said.
In a statement, Mr Mellor said he was making "good progress".
"Thank you everyone for your kind thoughts and messages. I hope at some point I will be able to thank you personally or through other communications channels."
Mr Mellor received serious head and facial injuries in the attack, north of Taihape, on December 11.
He had stopped a red Mazda 323 hatchback after police received reports of it driving erratically.
He underwent surgery in Palmerston North Hospital to treat lacerations to his face, head and body after the brutal attack.
Two youths, aged 14 and 18, appeared in Wanganui District Court last week facing charges of assault, unlawfully taking a motor vehicle and attempted arson. Police are considering laying further, more serious charges.
