Man in serious condition after a violent assault near Mt Compass

A man who was allegedly assaulted with a hammer is in a serious condition in the Flinders Medical Centre.

A man who drove himself to hospital after being allegedly hit with a hammer near Mt Compass is in a serious condition.

The 70-year-old victim from Nangkita, south of Adelaide, was allegedly robbed of cash and assaulted at his home with a hammer before being forced to drive at knifepoint to an ATM in Goolwa.

He alerted police of the incident just before 5pm on Thursday afternoon.

His alleged attacker, a 47-year-old Willunga South man, then drove him back home after he withdrew money from the ATM.

The victim then drove himself to the South Coast District Hospital for treatment, but was later flown to the Flinders Medical Centre after his injuries were deemed life-threatening.

SA Health say the patient is now in a "serious but stable" condition.

Police say the two men are "definitely" known to each other.

The 47-year-old man was arrested at his home just after midnight on Friday morning and has been charged with aggravated assault causing harm and aggravated robbery.

After being refused bail, he is expected to appear in the Christies Beach Magistrates Court on Friday.