Old NSW orphanage burns two nights running

An abandoned orphanage in NSW has caught fire for the second night running.

The roof of St John's Orphanage, in Goulburn, has collapsed and the blaze may have destroyed the building, which is more than a 100 years old, the NSW Rural Fire Service says.

Friday night's fire comes just 24 hours after crews saved the building from another blaze but the RFS cannot comment on the cause.