Adelaide's problem-plagued North Terrace tram extension has hit another delay after signalling issues were identified and driver training suspended.

Passenger services had been scheduled to start operating on Sunday but will now be delayed by at least a week, Transport Minister Stephan Knoll said.

“Testing has been ongoing over the past number of days but after this morning’s testing, in the early hours of this morning, it’s been confirmed that passenger services won’t be able to start on the 29th of July,” Mr Knoll said on Monday.

Mr Knoll said experts from Hanning and Kahl, the company that provided the tram signalling system, are flying to Adelaide from Germany to help identify the issue.

“There are a large number of electrical contact points and pinpointing exactly where this fault occurs has been difficult,” he said.

The signalling issue was identified during driver training along the new line, and training was then suspended.

Mr Knoll said the costs incurred in fixing the issue would be covered by the contractor instead of taxpayers, and apologised for the delay in completing the project.

But opposition transport spokesman Tom Koutsantonis said the delay raises questions over Mr Knoll’s credibility.

“We had a guarantee now on four occasions from this minister that it would be completed by a certain time and each time he’s broken his word,” he said.

Mr Koutsantonis said the previous Labor government refuses to accept any responsibility for the delay, despite failed efforts to finish the project before the election.

He instead blamed the set back on the sacking of the former head of the Department of Planning, Transport and Infrastructure, Michael Deegan, who, he said, had been on track to complete the project on schedule.

“We were told, before caretaker, that the agency could deliver this program and have driver training in place by March,” Mr Koutsantonis said.

“The first act of the new government was to sack the man in charge of this.”

By Kathryn Bermingham