We should not be surprised that the Australians had one 25pdr gun out of action by the afternoon of 21 April. The gun had an oil leak. The young gun commander was aware that the Germans had brought up infantry. On the left, they were unloading infantry. The Australian gunners were depressing the the guns and by firing "weak charges" so that they could fire at the infantry. The Australians fired some fifty rounds down the hill.
The German medium guns responded to the Australian fire. They also came under heavy machine gun fire that caused the Australians to take cover. When they came out after a while they realize that the remaining gun was inoperable due to a trail hit. The Australians had been engaged in an eight hour gun battle. None of the men where hit at that point.
The German medium guns now opened fire and caused casualties among the Australians. By dusk the Australians taking what they needed to take.
Once the decision had been made to withdraw from Greece, the New Zealand 6th Brigade was ordered to defend Thermopylae Pass. The battalions involved were the 24th, 25th and 26th Battalions. They had seven artillery regiments in support.
This is based on the account in "Greece, Crete, an Syria" by Gavin Long.