Newcastle City Council issued an urgent alert message about four hours after the road was closed, stressing that 14 ageing Hill's fig trees located between Darby and Auckland streets were not being felled.

The council resolved last month to remove the trees and replace them with a different species, because of public liability concerns.

But some councillors objected and the decision will be reviewed on December 15.

Community campaigns to save the trees are under way.

The council employed a consultant to examine the tree roots.

GBG Australia senior investigation engineer Val Donazzolo said a ground-penetrating radar was used to get an idea of where roots lay below the ground.

Mr Donazzolo said the device transmitted a radio pulse and waited for an echo to come back.

The time it took for the echo to come back indicated the depth of the fig trees' roots.

A spokeswoman said the council wanted to see if there were roots on the trees' north and south sides, as southerly wind posed the greatest threat to trees falling.

"The presence of roots on the northern and, most critically, the southern side of the tree are critical for stability," she said.

The information would be made available to councillors to help them make informed decisions regarding the figs.