A South African animal rights group is planning to sue the people responsible for the death of a giraffe that smashed its head on a bridge as it was transported in a trailer on one of Africa's busiest motorways.

Eyewitnesses reported seeing two giraffes in an open air trailer being driven along Johannesburg's N1 motorway on Thursday before one hit its head on the bridge.

"What a weird and sad thing to witness. That impact was so shocking," TV and radio personality Pabi Moloi said on Twitter, next to a cellphone picture of the giraffes shortly before the incident.

"Look how low that bridge is and how tall the giraffes are. Who thought this one through?"

South Africa's National Society for the Prevention of Cruelty to Animals said they would be seeking to prosecute those responsible.

"We're very concerned about the way it was transported. Most definitely there has been negligence," NSPCA manager Rick Allen said.

The incident has caused outrage in South Africa and has been tragically compared to a scene in Hollywood film 'Hangover 3' in which bungling character Alan Garner buys a giraffe before accidentally decapitating it on a highway bridge.

Allen said it was fortunate the other giraffe survived. "We have already nicknamed it 'Lucky'," he said.