Canberra Metro will also miss out on its first availability payment - worth millions of dollars - because of the delays.

With the final rail due to be laid on Friday - seven months after Transport Canberra's deadline - Transport Minister Meegan Fitzharris has moved to provide clarity on when the Civic to Gungahlin rail line will begin operation.

The original Canberra Metro contract says construction would be finished by late 2018, with service to start by early 2019, however it's understood there had been potential for the service to start running in late 2018.

That's no longer the case, Ms Fitzharris said on Thursday.

The project is understood to be several months behind schedule, with delays said to be caused by safety on site and the sheer complexity of the project.

Ms Fitzharris said while construction is still due to be finished in late December, Canberra Metro will need to obtain third-party rail accreditation from the Office of National Rail Safety Regulator and the Independent Certifier before the system begins operations.

“As with any complex infrastructure project, there are risks that affect timing such as weather, the testing and commissioning of rail and signalling systems, and finalising of the stops," Ms Fitzharris said.

“It is clear to all that significant progress is being made on the project, and I look forward to updating the community on a start date as soon as possible.

“While this means we don’t yet have an exact date, I am expecting the first services to begin in early 2019, which is really exciting."

It is not yet clear how long accreditation from the Office of National Rail Safety Regulator will take.

The regulator's 2017-18 annual report said following the initial granting of accreditation in September 2016, the office has been working closely with the operator on variations to their accreditation as they progressed through the construction phase to the testing and commissioning phase for both the rail infrastructure and rolling stock.

In May 2018, the operator applied to vary its accreditation for passenger revenue services, the report said.

The announcement comes after Transport Canberra's latest annual report showed the project missed nearly all of its milestones by several months.

One target, the full energisation of the network for testing, was due to be completed in May. While the overhead wiring system was completed earlier this month - 382 poles and 24 kilometres of cable - the network is yet to be fully energised. Sections on the northern end of the track have been electrified as light rail vehicles undergo testing.

The laying of the mainline track was scheduled to be finished by March but was 91 per cent complete as of June 30. The last rail will be laid at a ceremony on Friday.

When asked about the missed deadlines earlier this week, Transport Canberra said the consortium had "adapted its program ... to meet the dynamic needs of the project".

"It’s not unexpected that the program would adapt to meet the needs of what’s happening on the ground as light rail is being built," a spokesman said.

The delays will mean Canberra Metro will miss out on its first availability payment.

Under-Treasurer David Nicol told ACT Estimates in June the territory's $375 million capital contribution to the project would not be paid until operations began.

Availability payments each month to a total of $47 million for the first year will also be delayed until commencement.

The exact monthly payments are commercial in confidence, but they range between $2.8 million and $4.7 million a month.

"It’s quite a significant incentive for the consortium to start operations as soon as is safe for them to do so," Mr Nicol said at the time.

There is also an abatement scheme, which means the service must run to a certain standard.

"It is not just getting one tram running, so to speak. It has really got to be the full service at a good quality," Mr Nicol said.

Transport Canberra executives have previously said building the light rail safely was more important than finishing the project on time.

In documents obtained by The Canberra Times under freedom of information laws, it was revealed Canberra Metro was having challenges with recruitment and retention, with a number of staff leaving work on projects in Sydney and Melbourne.

"This is likely to have had a contributing effect on site safety and new resources are engaged on the project who may take some time to be fully trained in working with a tier one company and their safety management systems," Transport Canberra deputy director-general Duncan Edghill wrote.

“Transport Canberra through various forms has emphasised to Canberra Metro that ensuring a safe work site is the highest priority for the project. It eclipses all other concerns, including program."

Sydney's light rail project has also been delayed again with the deadline again pushed back to May 2020. It was originally supposed to be completed before the NSW state election next March.

That project has been tied up in legal battles, with Acciona suing the NSW government for $1.1 billion for allegedly failing to reveal that it had not secured the agreement of Ausgrid on how crucial cables under the route should be handled.

About 60 businesses have also signed up to a class action seeking around $40 million in compensation over disruption caused by the construction.