The director of ACT NOWaste admitted his department "took our eye off the ball" and there was a serious breakdown of communication between the government and their contractor Remondis.

But he said new processes had been put in place and the tip would soon have capacity until at least 2018.

The PriceWaterhouseCoopers report was commissioned by the government in the wake of the unexpected partial closure of Mugga Lane tip in October.

During the period, which lasted until December, half of Canberra's trash was sent through suburban streets to the re-opened West Belconnen Resource Management Centre.

According to the report, ACT NOWaste believed on their estimates the Mugga Lane tip had capacity until the middle of 2015, but a meeting with Remondis staff at the site in August revealed the dump would be full just two months later.

In their report, PriceWaterhouseCoopers said a lack of communication, a lack of proper processes and a failure of forecasting by ACT NOWaste had led to the miscalculation.

"There is no documented or consistent methodology applied by ACT NOWaste for the forecasting of landfill capacity at Mugga Lane," the report said.

According to the report, estimations of space available at the tip were being carried out by ACT NOWaste using historical data rather than the actual tonnage of garbage being hauled in.

ACT NOWaste director Michael Trushell said the issues which had occurred in the lead up to the tip closure were mistakes and would never occur again.

"Certainly when I arrived [in July] there was a degree of, you could see the communication wasn't going on, there wasn't that appreciation of how you develop the models," he said.

"We've fixed that now and we're moving forward on that. The model's been built, the methodology's there, I got an independent expert in to work with us fairly quickly."

Mr Trushell said inexperienced staff and a big workload were partially to blame.

"It was difficult under those circumstances to step back and have a look at the bigger picture and under those circumstances, yes, mistakes were made," he said.

Territory and Municipal Services Minister Shane Rattenbury said he was "deeply concerned" by the identified shortcomings .

"Across government there are a range of governance strategies and management approaches to avoid these sort of things, but clearly in ACT NOWaste those strategies weren't adequate and shortcomings have been identified," he said.

"Clearly ACT NOWaste did not have sufficient protocols in place to avoid these problems."

TAMS director-general Gary Byles sent a letter, obtained by The Sunday Canberra Times, to Mr Rattenbury which stated the directorate had already made significant changes in light of the report, including aerial surveying of the landfill by drones and an increased on-site presence.

"It is important now that the recommendations continue to be implemented to see ongoing improvement in the management and performance of the Mugga Lane Resource Management Centre," he wrote.

"As such, I have asked the executive director business enterprises and director NOWaste (sic) to brief me on a regular basis on the progress in achieving this outcome."

Correction: A previous version of this article stated the Mugga Lane tip did not accept landfill waste in the period from October to December 2014. This is incorrect - the tip was open throughout that period but the standby tip at West Belconnen received about half of Canberra's landfill waste during that time.