The dog was seized by police following the attack, which happened in Golcar Brow Road around 10.50am.
Neighbour Robert Dickinson used the shovel to try stop the dog.
“All I could hear was wrenching screams for help,” he said.
“I came out, saw him and ran back to get the shovel.
“It had pinned him against a fence.
“I hit the dog with the shovel but it wouldn’t let go. It was hungry for blood, you could see it in its eyes.
“It was some sort of Staffy.”
Neighbour Stacey Allan estimated that the attack lasted around five to 10 minutes.
“It was horrible. The dog had hold of his right arm.
“It was pulling him around and he was in agony and there was blood everywhere.
“We got it off and he fell to the floor.
“His arm was pumping with blood so we put belt around to try stem it until the ambulance came.
"He had to be taken into the ambulance and the police came and took the dog away.
“It’s shocking because lots of children play around here. Something needs to be done so this doesn’t happen again.”
A man, who would not give his name but was living at the house where the dog came from, confirmed the attack but said he was “only looking after it for a friend.”
He added: “I went to get the post and the dog must’ve jumped over the fence in the back garden.”
Police said nobody had yet been arrested since the 10.50am attack, but the dog had been seized. No description of the dog was given.
Royal Mail spokeswoman Morag Turnbull said: “Royal Mail can confirm that a serious incident occurred today in Golcar Brow Road, Meltham, when a postman was attacked by a dog in the street whilst delivering the mail.
“Police attended the scene and seized the dog. The postman has been taken to hospital and is receiving treatment to an injury to his arm. Our thoughts are with him and his family at this distressing time.
A spokeswoman for the Communication Workers Union (CWU) said: “The postman is a member of the union and safety reps and other union officials have been involved. He has apparently suffered very serious injuries including significant blood loss. He is at Huddersfield Royal infirmary having emergency surgery, I understand.
“We would like the law to be applied to both the owner and keeper of the dog.”
A recent change in the law means prosecutions can be brought in cases where dog attacks occur on private property as well as in public places.
Dave Joyce, CWU health, safety and environment officer, said: “This is a serious attack with serious injuries and we expect the owner and keeper of this dog to feel the full force of the law.
“Our thoughts are firstly with our member who has been seriously injured during the course of his job delivering the post. We will give him every support he needs, including through the criminal prosecution which we expect to be brought and also if he wants to pursue civil action.
“We expect the police to bring an appropriate prosecution following their investigation.”