WA should and can ask to guarantee its GST allocation will not be affected by health reforms especially the Commonwealth retaining 1/3 of GST revenue.
It can be done by a Federal Treasurer guarantee, through instructions to CGC in its assessments of GST sharing relativities. This can be written in the agreement.
That is perfectly logical, reasonable and realistic.Many things have been done that way, including senate political bargaining for particular states SPP in the past.
WA's case is much stronger, given that this is Commonwealth request, a change to intergovernmental agreement, and the PM has publically said no States will be financially worse off.
So, the WA Premier should be strong, firm and go for it. He will get it.
WA Treasury should be able to advise the WA Treasurer and the Premier on how this can be done.