Please help. Need advice for visa extension

Hi Folks,

Need your advice guys. I have a TB history. Everytime I apply visa, I will have to go through xray, sputum test which takes about 5, 6 months. I am currently holding 600 visa and going to expire soon. Thus, I am thinking whether I should stay in australia to extend or I should just apply new visa again from offshore. I have hap id where xray has already completed and no further request being asked to go through the sputum test but I am afraid they will ask me go through test upon submitting the visa and delay visa again. If that happens, by applying onshore, i still can get bridging visa.

I still do not understand how Australia health declaration works.
I am trying to get agent to give me the best solution.

Thanks in advanced.