The buffer would rule out residential development north of Gungahlin or anywhere along Canberra's northern boundary, except for the ACT Government's joint venue with the Corkhill family to develop Ginninderry, which is being given an exemption.

Landowners, many of which had planned future residential development, say it will devalue their properties.

The Yass council is set to adopt the new plan at a meeting on Wednesday.

Farmer Graham Darby, whose 240-hectare property joins the Mulligans Flat Nature Reserve, said Gungahlin was grazing land when he began farming in the area 40 years ago.

Suburban development had squeezed wildlife into 1200 acres of largely infertile rocky country in the nature reserve, and with no natural predators, the kangaroos were now devastating adjacent pasture in NSW and making it unviable for sheep grazing.

With his sheep farm declining in viability, he is part of the Oakstand group of landowners who want  to develop their land for housing.

But the Yass council's RU6 “transition” zone would stop residents submitting development applications for 20 years.

Mr Darby criticised the exemption for Ginninderry.

Landowner David Wicks bought his 130-hectare border property with business partner Steve Evans 12 years ago, and said the property was part recreational farm and part investment, with the expectation that it would eventually be able to be developed.

The buffer zone had come as a surprise and if it was adopted by the council on Wednesday landowners would consider a legal challenge, he said.

Mr Evans said he didn't understand the need for a buffer, given the council already had the power to approve or reject any development applications.

“Why would they want to tie up a strip of land so close to Canberra that's probably the most logical place for Canberra to expand into?” he said.

The pair are also part of the Oakstand group, which points out that Yass's water infrastructure will soon reach capacity, requiring a substantial investment in new pipelines.

The money could only come from big upfront payments, such as developer payments on new lots, and by adding “a large pool of Canberra workers to Yass Valley Council's revenue base” through development near the capital.

Light rail could also feed into development north of Gungahlin.

Pam Christian, who owns property on the border at Mulligans Flat Road with husband Jon, also questioned the exemption for Ginninderry, saying the barrier to other development near the border would protect Ginninderry sales and destroy the possibility of desirable and affordable land close to the ACT border for new homeowners.

The Christians are among landowners to propose six connected villages across 2250 hectares north of the border at Mulligans Flat, stretching down towards the Federal Highway.

Each would be home to between 2000 and 6000 people, with up to 27,000 people in all.

Another landowner accused the council of “offering my property as a preservation zone for nature parks within the ACT or a rural character gateway” to Canberra without compensation.

​And yet another described the proposal as “random, unequal, unfair and undemocratic”.​

Yass council planning director Chris Berry said the buffer was designed to preserve the separate identities of Canberra and the Yass area, and to stop ACT development “creeping over the border and up the highway” and “bleeding into our villages”.

Ginninderry was an exception because it was landlocked, with no access into the NSW other than over the Murrumbidgee River and Ginninderra Creek, and road access through the ACT.

The council was also considering tighter restrictions in the zone to ban intensive agriculture such as poultry farming, and uses as such as truck depots.

The ACT Government supports the buffer. It told the Yass council a buffer would maintain routes for animals and plants along the northern border, minimise the risk of ribbon development along the Barton Highway, and maintain a rural landscape on the approach to Canberra.

It has urged the council to ban rural subdivision below 80 hectares.