Firefighters have brought a large fire in a Masterton pub under control.

Former Masterton mayor and current councillor Gary Caffell posted about the fire at Farriers Bar on social media just after 8am on Saturday, asking people to stay clear of the area.

There were a number of fire trucks at the Trust House's Farriers complex, he said, "with flames and heavy black smoke billowing out of the roof area".

The Farriers Bar and Eatery complex is a distinctive landmark at the north end of the Wairarapa town.

Fire and Emergency NZ received an automatic fire alarm triggered at the bar about 7.30am, a FENZ spokesperson said.

"Further 111 calls reported a building fire. When firefighters arrived, the building was well involved and the response was subsequently increased to a third alarm.”

Just before 9am, FENZ said at least 10 crews were at or responding to the fire, including from nearby towns and two specialist appliances from Remutaka and Avalon fire stations, as well as resources from Wellington.

"The fire is located on the first floor of the two-storey building, with aerial operations under way."

The blaze had been brought under control about 10am, they said in an update, and aerial firefighting had stopped.

"Crews remain at the scene carrying out minor internal firefighting and salvage operations to protect unaffected areas of the building and its contents."