Falling short of the minimum 40% target for 2030 would mean Canada missing its commitment under the United Nations' Paris Agreement on climate change.
Ottawa's plan is insufficient because key measures needed to meet the 2030 target were delayed or not prioritized, the office of the auditor general said in a statement.
The audit, by Commissioner of the Environment and Sustainable Development Jerry, found responsibility for reducing emissions was split among multiple federal entities not directly accountable to Environment Minister Steven Guilbeault, making progress and course correction difficult.
"I agree with the commissioner, we need to do more. We need to do it faster," Guilbeault told reporters.
In the next few weeks Ottawa will unveil measures on zero-emissions vehicles, mandate a 70% cut in methane output from the oil and gas sector by 2030 and produce a framework for a cap on oil and gas emissions, he said.
Canada has missed every emissions reduction target it has ever set. Prime Minister Justin Trudeau says fighting climate change is one of his government's top priorities.
Canada last year released its first real roadmap to meeting 2030 climate targets, laying out detailed plans and C$9.1 billion ($6.6 billion) in new spending to cut planet-warming carbon emissions.
DeMarco said the government could still meet its 2030 target "with drive, focus, and leadership".
The audit found the plan had potentially strong measures, such as carbon pricing and regulations, but also had many weaknesses, "including missing and inconsistent information and unreliable projections that hindered the plan's credibility".
