Cannot modify the instance class because there are not enough availability

HI,

I am getting the following message when I try to upgrade by db.t2.small instance to a db.t3.small in the Sydney zone.

Cannot modify the instance class because there are not enough availability zones that have the requested instance class. Please try your request again at a later time. (Service: AmazonRDS; Status Code: 400; Error Code: InsufficientDBInstanceCapacity; Request ID: 371ac3cf-edd8-4308-8f3b-0279a15fb539)

Does anyone know why this is ? I tried it a couple of times over a couple of days.