In short and notwithstanding your general duty of care, it depends very much on the degree of control you have over their work. The more control you have over what they do, how they do what they do and when they do what they do, the greater degree of responsibility you have over their H&S. A lot also depends on whether or not they are using your tools and equipment or whether they supply their own.

Jodie.
Someone needs to be responsible for their H&S. Whether it is you or their direct employer, someone needs to be responsible for their welfare.

It all depends on their work, their competence/experience and the relationship between your company and the employer. If the employer is fully aware of the work which they are required to undertake (I assume they are) then they should supply them with all the required H&S info, as it is they who are ultimately responsible for the care of their employees. However this H&S info will probably have to come for you in the first instance.

Hi Jodie. Your site, you own the risks and the duty to reasonably control those risks, whether to your own employees, somebody else's or to site visitors.

Their employers have duties too but it still remains your site and your work activities that create the risks.

Hi Jodie,
I'd summarise your responsibilities as follows:

s(2) HSW74 - you have a duty of care towards your employees, i.e. those directly on your payroll

s(3) HSW74 - you have a duty of care to those not employed by the company but affected by the company's activities, i.e contractors/ visitors

As has been mentioned previously, the practical level of supervision you need to exercise will depend upon what they're doing, but, as I'm sure you realise, you can't just leave them to it and hope :0)

Maybe it would be helpful to draw up a specific risk assessment for these contractors - once HR have told you what they're going to be doing, of course!

Hope this helps :0)