Ourimbah is indeed on the trainline between sydney and newcastle, but it's not a major stop. You would have to catch an all-stations train from either gosford (coming from the sydney end) or wyong (coming from the newcastle end). Ourimbah station is about 10-15 mins from either option.

timetables at www.cityrail.info or as an alternative http://www.cityrail.info/