Police in Queenstown are looking for someone who was wet and cold when they arrived home on Sunday night or early yesterday, after a 50-tonne digger, worth $1 million, was driven into the Kawarau River.

Constable Jonathan Cochrane said the digger, which belonged to Steve Rout Contracting, was one of only four of its kind in the country and the only one in the South Island.

A man who flew to work over the river each day noticed the digger submerged in about 4m of water about 7am yesterday, Mr Cochrane said.

The digger was moved from its park below the Lower Shotover Bridge, in an area known as the Shotover Delta, between 4pm on Sunday and 7am yesterday.

It had been parked on a bank and was either hot-wired or started with a screwdriver, driven down a steep embankment and into the river. A steep drop in the riverbed meant it got stuck about 100m from the river's edge.

The person responsible would have had to swim to dry land, Mr Cochrane said.

"They had to be able to drive a digger. You have to be reasonably skilled to even drive the thing and they had to know what it was capable of to drive it down that embankment.

No one else would even attempt that."

Rout employees struggled all day yesterday to come up with a way to remove the digger, but it was proving difficult.

Last night, Stacey Rout said employees would build a road out to the digger and it would be lifted by two cranes today.

The cost of the operation was likely to be about $100,000 and it was not yet known if the digger could be repaired, or would have to be replaced.

"I didn't believe them when he was first notified. The foreman and I ... drove out to the site, came around the corner and saw the arm sticking out and my heart sank."

The business had been the target of juvenile pranks in the past, but Mr Rout said after the latest incident, the Shotover Delta area might be better off closed to public access.

Previously, damage included broken windows on machinery and the offenders would spend "a couple of Saturdays working off their debt".

"This might take a few years to pay off. I just hope the cops find them. Some sort of justice needs to be paid," Mr Rout said.

Police want to hear from anyone who saw suspicious activity in the area, particularly motocross enthusiasts who frequently use it.

- OTAGO DAILY TIMES