I'd hate to use the expression 'a needle in a hay stack' but that's what we're dealing with at this stage

Bob Fields, Australian police

Bob Fields, southern region commander of the Northern Territory police said: "I'd hate to use the expression, a needle in a hay stack, but that's what we're dealing with at this stage."

Mr Fields said that while tracking was not an "exact science" he was hopeful that the Aboriginals may succeed where police had so far failed.

Four-wheel drive

Police have closed off 300 square miles of road in an attempt to box off the gunman.

Motorists are being stopped and their vehicles searched by police.

But the Northern Territory covers a vast area stretching over a million kilometres, and it is feared the gunman may have slipped through the net in a four-wheel drive vehicle, quite capable of driving off-road.

Police also fear for Mr Falconio's safety after finding blood near the scene of the ambush.

The British couple, from Huddersfield, in Yorkshire, had been travelling through Australia as part of a round-the-world trip they began in November.