You may find that as the original GST legislation went through the senate it could be possible for the Feds to change it without the states asking.
Not sure about that Runner. The original Bill said that the 'Agreement of Principles' between the Commonwealth and States and Territories that was reached in 98 applied in the setting of the GST rate. That section was not amended when the Bill was enacted.
1‑3 Commonwealth‑State financial relations
The Parliament acknowledges that the Commonwealth:
(a) will introduce legislation to provide that the revenue from the GST will be granted to the States, the Australian Capital Territory and the Northern Territory; and
(b) will maintain the rate and base of the GST in accordance with the Agreement on Principles for the Reform of Commonwealth‑State Financial Relations endorsed at the Special Premiers’ Conference in Canberra on 13 November 1998.
Management of the GST Rate
31. After the introduction of the GST, a proposal to vary the 10 per cent rate of the GST will require:
a. the unanimous support of the State and Territory Governments;
b. the endorsement by the Commonwealth Government of the day; and
c. the passage of relevant legislation by both Houses of the Commonwealth Parliament.
Management of the GST Base
32. Subject to clauses 34, 35 and 36 of this Agreement, after the introduction of the GST, any proposal to vary the GST base will require:
a. the unanimous support of the State and Territory Governments;
b. the endorsement by the Commonwealth Government of the day; and
c. the passage of relevant legislation by both Houses of the Commonwealth Parliament.
33. All future changes to the GST base should be consistent with:
a. the maintenance of the integrity of the tax base;
b. simplicity of administration; and
c. minimising compliance costs for taxpayers.
Runner if you're interested the key section of the GST legislation is
The relevant Principles are: