Hi,
traveling to different time zones affects the circadian rhythm, indeed. The change of daylight and sleep patterns affects many systems of the human body, including the hormonal system.
If you take the hormonal system as an example, the hormone Cortisol is release by the adrenal glands in large amounts after awakening in the morning. If your sleep patterns are disturbed, this will also disturb your cortisol levels. Cortisol is believed to have an affect on the immune system, and another function is to help in the break down of fats lipids. The break down of lipids probably explains why shift workers are can easily gain a couple of pounds without chaining their eating habits neither there exercise levels. I have had that personal experience while I was working different shifts at the hospital. Anyway, the human body will always try to get back to homeostasis as quickly as possible.
From my personal experience, it takes around 5-6 days to get back to normal if you choose your flight times wisely. I went to a specialist homeopath when I was travelling a lot to support my body and she prescribed my some homeopathics which really worked for me and reduced the jet lag time to around 2 days. I always try to get a midnight flight, so I'm tired and sleep the first leg. Depending where you flying from, Australia is tough one. though. From Europe, a stop over in Asia for a day or two definitely helps to stretch your legs a bit.
Bye