Assistant Minister for Defence David Fawcett revealed the statistics and reasons behind the cull in a letter sent last week to Invasive Species Council chief executive Andrew Cox, who says it "shows the degree of planning that went into the operation and the limited options available". 

In the letter, Mr Fawcett says the Department of Defence undertook "a number of trials and feasibility assessments of both lethal and non-lethal removal options over a 10-year period", including re-homing horses and ground shooting from boundary perimeters.  

Both, however, were unsuccessful because of low capture rates, difficulty of the horses to be handled and the size of the inaccessible area. 

"Following the limited success of the alternative methods trialed, the Department of Defence determined that an aerial cull … was the most appropriate method for the humane control of the horses," Mr Fawcett said.  

The assistant minister says the cull was "conducted in accordance with environmental policy and relevant codes of practice" in order to "humanely remove a threat to both personnel using the training area and to the environment". 

The method of aerial culling, a process banned in NSW but able to be conducted on the Singleton base because it is federal land, was slammed by animal welfare groups in the lead up to the December 19 cull. 

The 235 horses killed was also more than the 150 the Department of Defence estimated to be on the base before the operation. 

Hunter Valley Brumby Association president Kath Massey, a vocal advocate against the cull, described the numbers as a "massacre".

"I'm shocked at not only the amount of numbers but the time it took," she said. "To be able to get that many horses in that short amount of time … I've got no doubt it was not fully humane."

"They were all in treed areas, so they were shooting them through trees," Ms Massey said. 

"It was just the massacre we knew it was going to be. 

"There's no way in that short amount of time, with that amount of horses, that you could get a clean shot into every single one of them. There is no way, it is just not possible." 

Ms Massey said she had sought information from Defence in recent weeks about the cull numbers but had been told it was not available for release as it was part of a senate estimates hearing. 

Defence responses to questions on notice from Greens senator Mehreen Faruqi put the cost of the cull, carried out by a single marksmen using a self-loading .308 calibre rifle, at $33,783.

The sex of the animals was not recorded, but Defence estimates based upon the average mob composition, suggested 170 mares, 50 stallions and 15 juveniles were killed. 

"That's a lot of horses to shoot from a moving platform," Ms Massey said. 

"To be able to do it from a helicopter, looking down on a moving animal through trees … that's not carefully making sure you've got a head shot - there's no way.

"Being the time of year that it was they would have targeted pregnant mares, there would of been foals involved, which originally said they were not going to target. 

Mr Fawcett says in his letter "no live horses have been sighted on the Training Area since the control activity".

Defence also told Senator Faruqi "there were no surviving feral horses".