All 12 trams on the city's inner west line were decommissioned in 2021 to fix cracks up to 30cm long in rolling stock.
Trams borrowed from Sydney's other light rail line allowed inner west services to resume in February at a reduced frequency.
NSW Labor on Friday said a 2019 report concluded components on the trams were expiring earlier than expected because of poor design and manufacturing errors overseas.
The report, by a global rail engineering firm engaged to provide independent advice, supported an earlier report about the defects by Alstrom, which maintains the inner west light rail line.
"It's clear the government knew back in 2019 that their Inner West Light Rail trams were beset by defects, faults and failures caused by poor design and errors from the overseas manufacturer," opposition transport spokeswoman Jo Haylen said.
"If the government took action in 2019, we may have been able to avoid the shutdown of the entire Inner West Light Rail service, which caused massive inconvenience for passengers at a significant cost to the public."
Transport Minister David Elliott has been contacted for comment.
His department says the independent 2019 report was "completely unrelated" to the cracking in trams, the Sydney Morning Herald reported.
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