A truck brought down nearly 300 metres of tram lines on St Kilda Road on Thursday, causing significant delays on several key lines.

The truck pulled down the section of line south of High Street just after midday, Yarra Trams spokesman Simon Murphy said.

"We think it was an oversized vehicle that's pulled down the overhead wire," he said.

"It's about 300 metres of wire, so it's a relatively big section."

The outage affected routes 3, 5, 16, 64 and 67 in both directions, with all routes diverting via High Street for several hours and replacement buses being called in.

Power was restored and tram resumed their regular routes about 3pm.