German experts are being flown into South Australia because there is a fault with the signalling on the new eastern North Terrace tram line — and the State Government cannot work out why.

The Transport Minister's office confirmed that the signalling fault caused a tram to accidently cross onto the new section of track at the intersection with North Terrace and King William Street on July 16.

Passengers were safely taken off and there were delays to services while the tram was moved from the intersection.

The fault means that driver training on the new line has stopped and services won't open as intended on July 29.

Transport Minister Stephan Knoll said since the fault was first identified last Tuesday, it has been a struggle to find the problem.

"Essentially what we're working to do is to try and pinpoint exactly where that fault is," he said.

Mr Knoll could not say how long the project would be delayed — only that it would be more than a week.

He said experts from Hanning and Kahl — the German company that designed the signalling system — were on their way to Adelaide.

"The project team has essentially concluded now that the simple rectification to this is no longer possible, that an easy fault to find has not been forthcoming so now we need to move to a different phase," he said.

Trams have been operating along the western end of North Terrace since October 2007. ( ABC News: Malcolm Sutton )

The signalling problem is the latest in a series of delays on the tramline extension project that was started under the Labor government and was originally due to open by the end of March.

"This is extremely frustrating and regrettable and I certainly share the frustration and anger that South Australians must feel in relation to this and do apologise for our part in what's occurred over the last few months," Mr Knoll said.

In opposition, the Liberals criticised the extension for omitting a right-hand turn to allow northbound trams on King William Street to turn and head east along North Terrace.

The party promised to build the turn, despite reports showing it would cost $37 million.

The Government has been reluctant to set a new date for that project, but says the latest delay doesn't affect those plans.