A standard sedan travelling at 40km/h could push one of Melbourne's new concrete bollards up to 30 metres, experts warn, raising concerns about their safety and effectiveness.

Melbourne City Council has installed 206 temporary bollards in 10 CBD locations as an urgent anti-terror measure to prevent vehicles ploughing into pedestrians.

But bollard industry experts say the blocks, which have been hired from the Harry the hirer company, are not designed to be used as barricades. Because they are not fixed to the ground, cars and trucks could push them into pedestrians in a crash or terrorist attack.

Harry the hirer usually rents the blocks as counterweights to tie down marquees. They weigh about one tonne.