Federal Environment Minister Sussan Ley was due to hand down a decision on January 13, but the timeframe to decide has been extended until January 29.

The ACT government referred the plans for the city to Woden light rail line to the federal Department for Environment Protection and Biodiversity Conservation approval in August 2019.

The decision due this month could give the go ahead for stage 2A - between Alinga Street and Commonwealth Bridge - of the light rail route to Woden.

The project is already facing significant delays with contracts yet to be signed and construction unlikely to begin in 2021 as previously proposed.

It also appears unlikely the route will be operational, as previously suggested, by 2023.

A spokesman for Transport Minister Chris Steel said construction timing depended on a number of factors.

"Major Projects Canberra has addressed all relevant requirements of the EPBC Act assessment process, and is not aware of any issues that would put the approval at risk," he said.

"The ACT government remains committed to progressing light rail to Woden as quickly and efficiently as possible through the various stages of planning, design, approvals, procurement, consultation and construction.

"Construction timing is dependent on a number of factors including the Commonwealth approval processes. These processes for both stages of the route to Woden are currently progressing in-line with technical design development."

The government decided to break up light rail stage two into two segments - 2A and 2B - due to the complex approval processes.

The first stage would run from Alinga Street to Commonwealth Park, with the second from Commonwealth Park to Woden.

Stage 2A did not require a full environmental impact statement but the government was required to provide preliminary documentation to the department.

Among the information required was how the ACT government intended to manage the potential destruction of populations of the critically-endangered golden sun moth.

This included moth habitat identified at the site of a planned "dedication traction substation" at Regatta Place, adjacent to Commonwealth Park.

The substation was proposed to help power the wire-free vehicles set to run on the new line.

Stage 2B will require more complex approval processes, including having to pass through Parliament and complete an environmental impact statement, due to its route through the Parliamentary Triangle.

The ACT government says it can start construction on 2A while it awaits approvals for the second leg.

The government is yet to release detailed costings of the project from Alinga Street to Woden.

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It released a business case last year on light rail stage 2A but key financial details were redacted.

The government says light rail stage 2A is estimated to return just 60 cents for every dollar spent on the project.

However it says once all of stage two is completed to Woden, the return for the whole stage will increase to $1.20 return on every dollar spent.