First check if on the ESS website, secondly recommend that it is checked over before installation (i.e. tell the customer that they could be wasting a lot of money if they get it installed and it doesn't work), we always advise availability of parts for second hand appliances i.e. can they still be purchased, how old the appliance is etc.  If it is not safe or fit for purpose then the gasfitter would be responsible.   No different in my mind than the things you have to do when putting in a new appliance.  You can't pass on responsibility to anyone else.

Some interesting changes to the Gas (Safety and Measurement) Amendment Regulations 2014 (2014/205).  See Regulation 75 has been replaced (Responsibilityof persons hiring or leasing gas appliances, fittings and gas installations) - replaced with Responsibilities of persons hiring or leasing out gas appliances, fittings, or gas installations or property or premises containing them:

Every person who hires or leases out any gas appliance, fittings, or gas installation or any property or premises containing a gas appliance, fittings or gas installation must:
(a)  Take all practicable steps to ensure that the gas appliance, fittings, or gas installation is safe at the commencement of the hire or lease; and
(b) ensure that the gas appliance, fittings, or gas installation is accompanied by instructions for its safe use, including information on any maintenance or ongoing safety inspections that are required.
(2)  For the purpose of subclause (1) (a)....

And 75A - Responsibilities of landlord for gas appliances, fittings, and gas installations at rented residential premises...

Well worth having a look at these changes (33 pages in all) - should be able to be downloaded from the ESS website or the Parliamentary website,  Came into force July 31 2014.