A Timaru tour coach driver fell off his seat attempting to retrieve a pen, causing his vehicle to plunge 40 metres down a steep embankment into West Coast bush, injuring 21 passengers.

Nathan John Hyslop, 32, pleaded guilty in Christchurch District Court today to 10 charges of careless driving causing injury after the crash on April 21.

Hyslop had an unblemished record as a driver for Kiwi Experience, a tour operator catering mainly to the backpacker market, and was unable to explain why he had reached for the pen, the court was told.

Police prosecutor Nigel Wolland said Hyslop had been working for Kiwi Experience for 3-1/2 years and regularly drove on the highway between Franz Josef and Fox Glacier.

Hyslop left Franz Josef about 7.45am on April 21 driving a 12.2 metre coach with 43 passengers aboard.

He was negotiating a "particularly demanding" 15km stretch of road known as the Fox Hills, where the steep hills and large number of corners carry a 35kmh speed advisory.

He was doing about 35kmh to 40kmh into a downhill S-bend when the pen fell from his dashboard onto the floor of the coach, Wolland said.

Hyslop reached for the pen as he was driving around a severe right-hand bend.

He leaned so far out of his seat that he fell, pulling the steering wheel to the right and causing the coach to cross the centreline and plunge down the bush-clad embankment, ending up on its left side.

Wolland said 21 passengers - mostly overseas tourists - were hurt in the crash, with 10 receiving moderately severe injuries such as fractured and dislocated limbs, cuts and whiplash.

Several suffered financial losses as well and reparations would be sought, he said.

Hyslop was shaken, expressed remorse and co-operated fully with police, but he was unable to explain why he'd reached for the pen.

Lawyer Pip Hall asked Judge Tony Couch for a remand until January for defence submissions. He said he'd discussed reparations with Hyslop and an offer would be made.

Judge Couch noted a large number of victim impact statements would be read at sentencing. He convicted Hyslop and remanded him at large for sentence on January 10.

Police withdrew a charge of dangerous driving causing injury.

- NZPA