1. The residential status for FY 2017-18 would be Non resident as you were in India for less than 183 days ( considering your stay period from April 1, 2017 till 4th sept 2017)
2. The residential status for FY 2018-19 would be Resident as you will be there in India for more than 183 days ( considering your stay period from August, 2018 till March 2019).
Now please refer the replies to the queries,asked in the first instance:
1) In India, your australian income will be taxed only in those financial years when you will qualify as resident of India. For the financial year when you qualify to be non resident, there will no taxability of income earned from outside India even when you remit them.
Given that you will qualify to be resident of India for FY 2018-19, the income earned during the tenure of Australia i.e. from April till you return India, will also be taxable in India as the global income. However, with the help of DTAA, India would be giving you the tax credits for the taxes already paid in Australia.
2) Since you will remain resident in India, in the FY 2018-19, you need not declare to bank as of now with respect to your residential status unless there are any changes in your plan of returning back.
3) The rate of 15% mentioned under TDS implies that the taxes will be deducted at this rate whenever you will receive such income. Definitely they have a relation to your actual tax slab rate. So, any income which is doubly taxed, shall be chargeable to tax as per the beneficial rate. By this I mean, if in the domestic tax laws, the rate of tax is 30% with respect to any nature of income and for the same income DTAA specifies any beneficial rate of taxation, then you may take the advantage of beneficial rate applicability.
Further, any taxes deducted under TDS provisions, you will be entitled to take the credit of such taxes at the time of filing the final tax return.
Hope this helps !
Should you have any other queries, please feel free to mention.