From the current Certified Agreement...
4.3.1 We expect that you will work a reasonable amount of additional hours if the requirement becomes necessary. You may choose not to work additional hours in circumstances where the working of such hours would result in you working hours which are unreasonable after consideration of:
(a) Any risk to your health and safety;
(b) Your personal circumstances including any family responsibilities;
(c) The needs of the workplace;
(d) The notice given by us and by you of your intention to choose not to work the additional hours; and
(e) Any other relevant matter.
There is no definition of "reasonable amount" in the agreement. ATC's are just exercising their right to refuse additional hours as allowed for in this clause.