Once the lease has been entered, it becomes rateable.
Until then its exempt as a public place under s556(1)(a).
A public place is defined in the dictionary as
(d) public land or Crown land that is not:
(iv) land that has been sold or leased or lawfully contracted to be sold or leased,
Public Land is defined in the dictionary as
… any land (including a public reserve) vested in or under the control of the council, but does not include:
(a) a public road, or
(b) land to which the Crown Land Management Act 2016 applies, or
(c) a common, or
(d) a regional park under the National Parks and Wildlife Act 1974.
Once its leased any exemption as above ends.
You would want to make sure the lease provides for the leasee to pay rates, taxes etc. I rather not do this – but if its real a deal breaker (paying the rates), Council might want to consider a donation/subsidy under s356 – all depends on what community benefit value the development is expected to bring.
Regardless of who pays, I reckon its rateable.