PETER CAVE: The main road linking the Top End of the Northern Territory to southern Australia is expected to re-open later today after serious flood damage on the Stuart Highway.

But it could take months before it's known what kind of environmental damage has been caused by toxic copper concentrate spilt when a freight train was derailed by the floodwaters.

The derailment and spill has prompted questions over the safety of transporting toxic substances including uranium during bad weather in the tropics.

Michael Coggan reports from Darwin.

MICHAEL COGGAN: Forty-eight hours after floodwaters swept part of a large freight train into the Edith River and cut off the Stuart Highway north of the Top End town of Katherine, several investigations are underway into the cause of the train derailment and the environmental impact of tonnes of copper concentrate spilled into the flooded Edith River from the freight train.

Jim Grant is the head of the Northern Territory Environment Department.

JIM GRANT: Well it's not a highly substance but it's not be ingested or inhaled. We think it's washed all over the place.

MICHAEL COGGAN: Stuart Blanch from the Northern Territory Environment Centre says the investigations also need to establish why the freight train was allowed to cross the river while it was flooded.

STUART BLANCH: The onus is on the owner of the railway line, Genesee & Wyoming. They should not have allowed that rail truck to proceed across the bridge. Either their procedures are not in place or they are not adequate or they were not implemented. It's got to be one of the three.

MICHAEL COGGAN: Genesee & Wyoming Australia owns the Alice Springs to Darwin railway. 
Managing director Bert Easthope says the company tested the track before the freight train crossed the bridge.

BERT EASTHOPE: We waited until we were given the all clear by the Bureau of Meteorology in the context of the cyclone warning. We did some physical inspection of the track and we had, to ensure the integrity of the track, and we also ran another service prior to that, that train going over that particular bridge.

MICHAEL COGGAN: Environment Centre director Stuart Blanch says the accident shows why dangerous goods including uranium oxide should not be carried on the Alice Springs to Darwin railway.

STUART BLANCH: A much bigger risk to Top End rivers would be derailment of trains carrying uranium oxide from the Roxby Uranium Mine in South Australia. The Territory Government should call a halt on approving the transport of more and more trains of uranium oxide up to Port Darwin until there are really adequate, safe measures in place to stop such a derailment ever happening with uranium on the track.

MICHAEL COGGAN: But Chief Minister Paul Henderson doesn't want to speculate about the implications for the transport of uranium in the Northern Territory.

PAUL HENDERSON: What we have had is a significant event. We've got the Australian Transport Safety Bureau conducting an investigation. Out of that investigation will come requirements for the operator to put in place and any recommendations in terms of government regulation we'll also accept.

PETER CAVE: Northern Territory Chief Minister Paul Henderson ending that report from Michael Coggan.