It's free between the hours of 6pm and 8am. Unless it is stated otherwise on the signs, in which case it's subject to the period (and days) identified.
There are a number of ways around this however, all pretty much involving the sign being made to not be visible within 50m iirc.
Otherwise you can just park on yellow lines and paint over the lines by your car with black paint (don't get caught, the fine is fairly sizeable).
Just a note that this is only for parking spaces on council property, private land they can almost do what they want.