NSW Transport Minister Rob Stokes says that amount of time is the "worst-case scenario" and he expects to have the problems rectified and the service back on track sooner than that,. But until then "the safety advice has been for these vehicles to be withdrawn from service until they can be fixed".

A replacement service has been in place this week and is already transporting 5000 passengers a day, Mr Stokes said.

About 1000 similar trams are in use around the world, as well as in Newcastle and Canberra, and Mr Stokes said the cracking that was found during routine maintenance by Transport for NSW is likely a design flaw that will have global implications.

"A lot of countries will be looking at what happens here with keen interest," the minister added.

He is advising global operators to "let them know this same design flaw is likely to be a challenge for their systems as well".

Transport for NSW chief operations officer Howard Collins visited the Lilyfield depot on Friday morning to inspect "and understand what we've discovered", including cracks up to 30cm long on the wheel arches.

He said the trams were not unsafe but "if the cracks propagate further it could be an issue" for the seven-year-old trams, and it will be "no quick fix".

"We want to fill a workshop with these trams with very competent engineers from Australia who can retrofit, strengthen and sort out these cracks permanently," Mr Collins said.

Labor transport spokeswoman Jo Haylen said "the government's transport procurement policies are in tatters" given "every single overseas built tram the government bought has had to be decommissioned".

"They bought trains that don't fit the tracks, ferries that can't fit under bridges or operate at night, and an entire fleet of trams that simply don't work," Ms Haylen said.

Thousands of passengers would experience delays and unreliable services until the trams were fixed, and the discovery of the cracks came just as people were beginning to return to work, she added.

Australia should start building infrastructure again to get a quality product and value for money, Ms Haylen said.

Rail, Tram and Bus Union NSW tram and bus president Daniel Jaggers said the government must offer alternative employment for workers on the inner west light rail.

He said the union "want the (government) to take full responsibility" after ignoring calls from transport workers for infrastructure to be made and maintained in Australia "for decades".

"Ever since the NSW government turned its back on Australian manufacturing ... it's been one disaster after another," Mr Jaggers said.

"The biggest losers are the people of NSW, who don't receive the public transport services they need and fork out billions in taxpayer's money to rectify the government's mistakes."

Inner West Mayor Rochelle Porteous is demanding the government provide free alternative transport.

"The inner west is paying the price of foolish decisions," she said

"This is a disaster of the NSW government's making and it's up to minister Stokes to fix it."

Australian Associated Press