Back then the country used the old Imperial Honours System. This was a British Empire wide system of honours and awards. The system had been developed to meet the needs of another country, and never was in accord with the way that Australians wanted an honours system to work.
A key British concept was that an award should reflect the social status of the recipient. In military terms, this was equated with rank. So an officer and a sergeant who performed the same deed together would receive different awards. The only exception was the Victoria Cross, which everyone was eligible for. Enlisted men were eligible for the Military Medal, while junior officers received the Military Cross.
Officers of the rank of major and above were eligible for the Distinguished service Order (DSO). The papers have described it as "second highest award for gallantry after the Victoria Cross". For field officers it could be. However, it was more commonly awarded for leading a unit during a distinguished action. Occasionally it was awarded to staff officers for good staff work. The award of the DSO to two field officers for the battle of Long tan is therefore unremarkable.
Being an Empire-wide system, the Imperial Honours System had quotas for each part of the Empire. This was particularly galling during the Korean War, when the British Army's inability to fill its ranks led to a heavy reliance on the Canadian and Australian contingents, but the quotas system ensured that the British Army still got most of the awards. In Vietnam, the same still applied, but Australia and New Zealand were fighting while Britain was not. many awards could not be approved because of insufficient quota.
Perhaps the most remarkable feature of the Imperial Honours System is that it lasted so long. Canada created its own honours system in 1967, and Australia followed suit in 1975.