Hf frequencies
Use high frequencies during day. Low frequencies during night.
Use high frequencies to call far away places and low frequencies to call close radios.
This all is related with the ionosfere. The HF waves uses reflection (or refraction-don't know much about that) At night we only have one layer. With day light we have two layers.
So, to call sydney I would use the higher one(8967) as it must be daylight there and Auckland 5643 as it must be already night.
a) 8.867 MHz 5.643 MHz
Hope this helps.
cheers