Relative humidity is the ratio of the amount of water vapour in the air to the amount it could hold if it was saturated. As temperature increases, the amount of water vapour the air can hold increases. Therefore, relative humidity decreases with increasing temperature assuming the amount of water vapour in the air stays the same. As a result, relative humidity goes through a diurnal cycle inverse to temperature, peaking in the early morning and reaching a minimum during the afternoon. For this reason, it is not the best guide to the amount of water vapour in the air - that would be the dew point.
17:13 EDT Canberra has begun its run of warmer than average days, with a few more on the cards.