NSW police blocked off Bungendore Road to traffic for most of Sunday as they hunted the alleged intruder who is believed to be on the run after stealing firearms and ammunition from the property.

Superintendent Rod Smith said the couple – believed to be a 74-year-old woman and her 76-year-old husband – were at home on their farm in Bywong when they heard a noise at about 11.45am on Sunday.

The couple went outside and were confronted by a man, believed to be his early 20s, dressed in all black and wearing pink socks with no shoes.

He was armed with a hunting knife and a hammer.

"He's gone into the house, held the couple there for some time, demanding keys to a gunsafe," Supt Smith said.

The couple were held up for about 30 minutes and the alleged intruder at one point hit the man on the head with the handle of the knife, causing a minor injury.

The man did not require any medical treatment for his injuries.

Supt Smith said the alleged intruder threatened the couple and managed to get the keys to the gunsafe "before directing them out of the property".

The couple managed to get to a neighbour's house and raise the alarm.

Supt Smith said the couple had been left "shaken and in a bit of shock".

The Bungendore property was in police lock down for much of Sunday, as officers hunted for the alleged intruder who is believed to have stolen a 22 rifle and a shotgun with ammunition.

There was a heavy police presence on Bungendore Road late Sunday afternoon.

The alleged intruder is described as 5'8 to 5'10 tall, skinny, Caucasian with short black hair. He was wearing all black with pink socks and no shoes.

Police are also searching for a silver hatchback seen in the area around the time of the break-in.

"It was possibly driven by a female or occupied by a male and female," Supt Smith said.

Anyone with information is urged to contact Queanbeyan Police on 6298 0512 or CrimeStoppers on 1800 333 000.