Justice Robert Hope Park in Watson will become a new nature reserve, while three hectares will be cleared nearby for medium density housing. The Pinnacle Nature Reserve in Belconnen will also be extended. Both additions will be made if changes to the Territory Plan tabled in the ACT Legislative Assembly are accepted.

In a submission on the proposed change, the Commissioner for Sustainability and the Environment, Dr Kate Auty, said the ACT government itself acknowledged significant challenges in meeting future offset requirements.

"Due to the large proportion of land in the ACT already committed to biodiversity conservation via nature reserve or national park designations, this increasingly limited supply means potential for contention over land use decisions made by the ACT Government," she said.

Planning and Land Management Minister Mick Gentleman said the additions would provide better protection to species and ecological communities. The Australian government had approved them as offsets for new developments that would affect threatened plants and animals.

"Territory Plan Variation 349 sees the addition of 19.3 hectares to the existing Pinnacle Nature Reserve near Weetangera to offset the impact of the development of the University of Canberra Public Hospital, including 7.6 hectares of white box-yellow box-blakely's red gum grassy woodland and derived native grassland," Mr Gentleman said.

"The variation changes the zoning of the 18.6-hectare Justice Robert Hope Park from urban open space to hills, ridges and buffer with a nature reserve overlay. The park is dominated by large, old prolific trees that provide valuable habitat for canopy-dwelling birds and woodland fauna."

Conservation Council ACT Region executive director Larry O'Loughlin said local park care groups had already well protected, restored and managed Justice Robert Hope Park. "To claim it is a biodiversity offset for clearing the adjoining three hectares is nonsensical and the loss of large mature trees cannot be offset," he said.

"Biodiversity offsets need to be additional and result in net gain to our conservation estate. The reality is that we had 18 hectares in good shape and well looked after and now we are going to lose three hectares of our locally and nationally significant box-gum grassy woodlands – a critically endangered ecological community. There is no biodiversity gain."

Mr O'Loughlin said conservationists were yet to be convinced biodiversity offsets worked whether in the ACT or elsewhere.

"We propose a better approach is to plan to ensure no development impacts on threatened species habitat and endangered ecological communities, while also allowing for adequate buffers and habitat connectivity to protect those ecological values," he said.

Mr Gentleman said the park would offset the impact from a three-hectare medium-density development of a neighbouring block in Watson and the extension of Negus Crescent to Antill Street at Prime Television, which has white box-yellow box-blakely's red gum grassy woodland and derived native grassland.

Mr Gentleman said ACT woodlands were some of the biggest, best connected, most botanically diverse examples of their typeremaining in Australia. But Mr O'Loughlin said over the past 10 years the ACT had lost more than 300 hectares of critically endangered yellow-box red-gum woodlands to urban development.

The tabling of the variation follows community consultation. Four submissions were received, which included supportive comments, and requests for more detail about managing the nature reserves.