To supply the aircraft to Ukraine means training, support and parts supply, which the Australians are probably unwilling to provide because a) the manpower and expertise to do so has been dispersed/wound down, and b) the cost and inconvenience of regenerating all that. It probably works out cheaper to scrap and bury, as the govt wants to do.
Airbus (NHI) also will want input into maintaining serviceability and therefore agreements with Ukraine will need to be made. Passing on modern combat aircraft to a foreign power that doesn't operate that type is not an easy process, particularly if the nation disposing the stuff doesn't hold design authority.
The entire MRH-90 programme was poorly run from the outset and blame, if any is to be directed at the Australian govt for bungling it from the outset. Obviously Airbus supply issues are responsible for part supply, but much of the criticism of the programme failing has been levelled at the govt procurement process and the ADF requirement and sustainability side of things, i.e. home-made problems. The ADF didn't want the MRH-90 to begin with, but the govt of the day ordered it over the ADF's preferred aircraft. Bearing in mind the muddled history of the programme and how many hoops Ukraine might have to jump through to get the helicopters, Ukraine might not actually want them...