MELBOURNE'S newest trams will be withdrawn from service for months from the middle of next year to deal with structural problems that have led to cracking in tramcar frames.

Operator Yarra Trams last night confirmed that microscopic cracks had been identified in two of Melbourne's 59 Combino trams and all would be taken off the road for months for repairs.

Revelations about the faulty trams, manufactured by the German-based Siemens, follow yesterday's confirmation of problems with Siemens trains.

Two years ago, the manufacturer's head office in Munich announced that the Combino trams had a structural flaw that in Europe had led to hairline fractures in the joints of aluminium bodies and made them vulnerable to collapse in accidents.

It ordered that all Combino trams, including those in Australia, that had reached the 120,000-kilometre mark should be recalled and modified. At the time, a Siemens Australia spokesman confirmed the Combinos would require modification.

Two years later, up to 10 of the Combinos have done 200,000 kilometres, all have passed 120,000 kilometres, and none has been withdrawn and fixed.

Melbourne Siemens spokesman Brad Voss said the company had decided the Melbourne Combinos were under less stress than those in Europe. "Our network is very flat and straight and we only operate three and five-car trams … they haven't exhibited any problems at all."

But Yarra Trams chief executive Dennis Cliche said Melbourne was as hard on trams as any European city.

Transport industry sources last night speculated that Siemens' attempts to play down the Combino problem in Melbourne might be related to forthcoming tenders for $1.3 billion in contracts for new trains and trams.

Melbourne University transport lecturer Dr Paul Mees described the problems as a "scandal" that exposed a lack of reserve trains and trams.

"We pay private operators double the subsidy we used to pay the Public Transport Corporation. They tell us we should be grateful because we now have new trams, but it turns out that the trams are duds."

He said it raised questions about who was looking after the public interest under a privatised system.

Mr Cliche stressed that the tram problems had never threatened passenger safety.

Matt Nurse, a spokesman for Transport Minister Peter Batchelor, said there was no comment because the Combino trams issue was "operational".

But he said he did not know the trams were being withdrawn from service next year.