For the best definition of what a SMS is you cant go past my friend wikipedia's definition "Safety management system (SMS) is a term used to refer to a comprehensive business management system designed to manage safety elements in the workplace"
Whether a contractor, employee or site manager you should have a good understanding of the system in place on site. As a contractor it is usually a mandatory requirement to have a SMS and everyone on site will be working with the system in one way or another.
Part of the system requires Job Safety Analysis aka JSA's, these are task specific. As you can see the one below is for unloading and loading trucks. For other tasks such as erecting structural steel, operating plant etc they would have their own individual JSA. The idea is for the workers to review the JSA prior to commencing work, to be made aware of the dangers inherent in the task and to use the control measures that minimise harm and risk.
I personally think its best to go the DIY route as SMS require updating and amendments and if you designed the system then it is quite easy to make changes, also no matter how good a safety consultant you use no one understands the work as well as the people doing it.