A funeral director has described how he frantically tried to stop an elderly couple's car before it crashed in a Sydney cemetery, killing one of them.

The couple, in their 80s, were leaving a funeral at Field of Mars cemetery in Quarry Road, Ryde, about 1pm on Friday when their Nissan Pulsar began revving, witnesses say.

It is understood the accelerator jammed, sending the car into reverse before it hurtled forward about 250 metres and slammed into a palm tree.

The woman died instantly while her husband was taken to Westmead Hospital.

Funeral director Michael Manning said he made a futile attempt to stop the car by reaching through the car window to remove the keys.

"Mate, he went backwards here ... I tried to get to the car to turn it off, and I got the door open and unfortunately, mate, it was in drive and he ... down he went," Mr Manning said.

He said he followed the car to where it crashed, but that the woman had been killed.

"Could you believe how I feel at the moment... when I was that far from saving somebody's life?" he said.

"Mate, it's, it's not easy ... I never got it and that's one thing, I've got to say, probably, I'll live with.

"But, unfortunately, I did me best."

Several horrified mourners witnessed the accident.

The accident appears to be the driver's fault but police are investigating.