The island is 150 k long and between 900 m and 57 k wide , its area covering 4405 k. Its coastline is 540 k long and highest altitude is 307 m. The island is 150 k long and between 900 m and 57 k wide , its area covering 4405 k. Its coastline is 540 k long and highest altitude is 307 m. It is separated from Yorke Peninsula to the northwest by Investigator Strait and from Cape Jervis to the northeast by Backstairs Passage .