Firefighters have searched the ruins of a building that went up in flames in south Auckland tonight and are confident nobody was trapped inside.

The abandoned homestead at 161 Robertson Rd on Mangere East is believed to have been set alight deliberately, resulting in a fierce blaze that took several hours to get under control.

Emergency services were called just past 8pm to the two-storey building at 161 Robertson Rd, on the edge of Mangere Centre Park.

There were initial fears that children may have been in the building at the time of the blaze. The Herald understands fire crews who were searching the house were forced to withdraw after the fire became too fierce.

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Fire and Emergency NZ spokesman Scott Osmond said the fire was now well under control and firefighters were damping down hot spots.

"They've completed a secondary search and there's no sign of anyone in there," he said.

The fire is being treated as arson. A fire investigator is at the scene and will be returning in the morning, Osmond said.

He believed the "big old two-storey house" was part of the park and may be council owned.

About 15 fire trucks attended the blaze, Osmond said.

One witness told Fairfax he saw boys running away from the scene.

A police spokeswoman confirmed three youths had been seen in the area before the fire began.

"We have caught up with them and are speaking with them," the police spokeswoman said. There was no indication they were responsible for the fire.

Local resident Timothy Ah Chong said he was first alerted to the blaze when he heard the police Eagle helicopter overhead.

"I heard the helicopter going over the top of my house and then I looked out the window and saw the smoke coming out," the 19-year-old said.

He could see at least three fire trucks and flames billowing from the building.

"We're not allowed to get close to the house - the firemen and police told us to move away," he said.

"The house has been left empty - nobody lives there, so that's good."

A crowd of locals had gathered to watch as the building burned, he said, including a number of children and teenagers.