Since then queues have been forming as people gather to see the mural which at first looks like a boy in the snow.
But on the other side of the garage wall the painting shows a fire in a skip, revealing that what appeared to be snow is actually ash from the smoke.
The painting is believed to be a statement on the industrial town, its steel works and the pollution it creates.
Banksy revealed he was the artist behind the mural on Wednesday in an Instagram video entitled Season's greetings.
The local council has now put up metal fences and on Saturday placed a perspex screen over the painting as crowds come to see Banksy's first piece in Wales.
Traffic wardens were also brought in to manage crowds in the quiet suburban alley, with councillors reminding visitors to not disturb those living nearby.
Hollywood actor Michael Sheen has even reportedly helped to help fund the security and legal bills to protect the work.
The 49-year-old, who is from the adjoining village of Baglan, wants to ensure the garage's owner does not have to pay to protect it.
Ian Lewis, a steelworker who owns the garage, said he saw the artwork on Facebook before he realised it was on his garage wall.
"At first I didn't really think much of it. I didn't even know if it was my garage. But then I had a look and saw it was," the 55-year-old said.
"It started off as fun, but now it's gone to stress. It's down to not sleeping, but now I've got the security there it's taken that weight off me.
"I've never experienced anything like this. My phone is ringing, on my house phone there's 1,000 messages on it."
He said he thought Port Talbot was chosen because the town made the news in July when black dust from the steelworks covered houses and cars.
And he said he had "no idea" what his plans for the artwork were and denied receiving any offers to sell it.
"I'm really just rolling with it at the moment and I don't know where it's going," he said.
Rachel Honey-Jones, 33, who lives the other side of Swansea Bay, said an artist friend of hers was tipped off about the artwork's location and stayed overnight to guard it.
"It's amazing, an incredible addition to Port Talbot," she said.
"People have already taken sledgehammers to it and tried to throw paint on it.
"It will bring visitors and trade and tourism to the county, so it really does need to be protected."