The underspend includes $79 million worth of construction works and up to $41 million worth of demolitions the taskforce did not complete last fiscal year, partly due to delays accessing land at several community facility-zoned and greenfield sites.

Given the delays and significantly underspent budget, it is unclear why the taskforce earlier this year decided to sell 33 of the townhouses it had built for public housing in Gungahlin to Defence Housing Australia, the key financial details of which the government has refused to release.

It follows an audit of the taskforce's work, which is replacing 1288 old public housing dwellings to generate funds for light rail, last year finding a series of bungles on the project, including pushing public housing tenants to outer suburbs in conflict with a June 2014 Cabinet submission.

While the government has confirmed it rolled over $79 million in unspent capital works funding from last fiscal year to 2018-19, it has not confirmed a specific figure for the taskforce's unspent supplies and services budget, though the planning directorate's annual report shows $41 million rolled over was primarily due to the taskforce.

Of the $114 million the environment, planning and sustainable development rolled over from 2017-18 to 2018-19, $120 million was largely attributed to the taskforce.

Overall, the directorate underspent its planned $100 million supplies and services expenditure for 2017-18 by 41 per cent, spending only $41 million, and underspent its total $327 million capital works budget by 32 per cent, spending only $222 million.

The taskforce has also canned plans to demolish the buildings on existing public housing sites at Strathgordon Court in Lyons, Stuart Flats in Griffith and Gowrie Court in Narrabundah itself, instead making developers that buy the blocks clear the land.

The government will make it a sale condition that buyers demolish all existing structures on the site, consistent with development applications already approved for each block, with all three sales and settlements expected by June 2019.

It is unclear why the specific change was made to the demolition schedule, as the taskforce would not answer such questions, but it appears to be a change to ensure settlement can occur to meet the Commonwealth's June 2019 deadline, so the territory can get expect bonus payments under the federal asset recycling scheme.

But a directorate spokeswoman did confirm demolitions on the three remaining sites may now not occur until the end of the 2020-21 financial year, which could leave the public housing buildings vacant for several months after settlement.

The spokeswoman said that once all evidence to demonstrate a successful demolition process was provided to government, the demolition contribution for those blocks would be paid.

"These payments are expected either in 2019-20 or 2020-21, depending on demolition completion," she said.

Separate to the demolition schedule changes, the spokeswoman said the taskforce would deliver all 1288 dwelling it had promised by the June 2019 deadline, except the proposed development in Holder, which is currently subject to an ACT Civil and Administrative Tribunal appeal by local residents.

"The taskforce continues to work with the relevant stakeholders to progress this process," she said.

"However as this is a legal process, no further public comment can be made on the proceedings at this time."

But the taskforce has also abandoned plans for public housing at Greenway Section 10, with the spokeswoman saying that site was "not pursued due to not being able to meet the renewal program time frames".