Landlord's can pretty much choose to charge whatever they want to - it's their property. Unless they are trying to change it while a current lease is still in force, in which case they would be in breech of contract, you don't really have any say if they want to raise it.
In Australia they can put it up every 6 months if it's written into the lease or if your not on a lease. Once upon a time they had to make improvements to warrant putting up the rent but nowadays they can Do it without lifting a finger. I always try for a 2 year lease but it's hard to get it. Most do a one year lease. You can write a letter explaining it will cause hardship or you as I did & it only went up by $5 a week instead of $10 as they had wanted. So a compromise can happen. Ive been living in this house for nearly 3 years. You can ask for anything that needs fixing to be fixed if they are upping your rent. You can ask even if their not, but if they are you want your money's worth for your rent so make a list & present it to them nicely. Goodluck