Power prices are on average higher during evening (and morning)...

Power prices are on average higher during evening (and morning) peak periods, the amount varies based on many factors, including the amount of coal and gas generation capacity available at a given time, and the current output from wind generation as determined by the weather.

Selling power on the spot market is not the only possible source of income for Kidston hydro - they might also have PPAs, firming contracts, and provide FCAS services. There are also sometimes periods where there is excess power in the grid, e.g. when there's more than anticipated wind and solar generation, or an interconnector might trip causing excess power on one side. During such periods, power prices might be negative and Kidston might get paid for providing load that is used to pump water into the upper reservoir.