The theft of a fully-grown emu from an Australian wildlife park this week has left only a pile of feathers at the scene of the crime, and questions about the motive for snatching an ungainly bird with practically no cash value.

Operating under cover of darkness, robbers are believed to have lifted the flightless bird - second only to the ostrich in size and known for its speed, powerful legs and clawed feet - over electrified barbed wire atop a two-metre fence, eluding a guard and a security camera.

Police evidence suggests a getaway vehicle was parked about 1 km (0.6 miles) away near a train line adjacent to the park, said Chad Staples, senior curator at Featherdale Wildlife Park in Doonside, west of Sydney.

"It would have had to be carried the whole way and lifted over the fences twice," he said. A grown emu can be as much as 2 metres (6.6 feet) tall and weigh roughly 37 kg, or a little more than an adult Labrador dog.

All that remained in the enclosure was a heap of feathers. A second emu was also in the area, but it escaped the thieves with minor feather loss.

Staples said he was mystified by the theft, the first of its kind.

"Emus don't really have a monetary value because of how common they are," said Staples. "It (the theft) was extremely targeted and it seems fairly well executed."

The last break-in at the park was on Christmas Day in 2012, when 10 macaw parrots were stolen but recovered shortly after.

"We are hoping the same thing will happen with the emu," Staples said.