'Working to rule' is normally the first stage of strike action. Not sure about the impact on Mount Cook, but Eagle certainly aren't able to operate properly without pilots accepting call outs. I gather for the MC boys and girls, this means not being available for work on their 5th day off. Given that they don't roster standbys, this could be very tricky for the company.
This is the point when the company typically withdraws staff travel 'privileges', and possibly tries to cancel leave.