This question is from Nic (paraphrased to suit this format):
If I serve a 90/63 day notice under s51, can my tenant return serve a 28-day notice to bring forward the termination?
Yes. The tenant’s ability to issue a 28-day s51 notice to terminate is unfettered even if there is a (landlord initiated) termination notice already in place. Should a tenant return serve such a notice, the tenancy terminates on the earlier dare per that notice.
In the alternative, the landlord and tenant can also mutually agree on a termination date (that falls outside of the statutory minimum notice periods). As long as that agreement is properly documented, the tenancy will terminate as agreed.
Landlords should be aware of and make plans for the likelihood of termination notices being shorted on these bases.