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I am assuming you are asking about the Transit (subclass 771) visa. You will find the actual regulations on this visa in Part 771 of Schedule 2 to the Migration Regulations 1994.
- Does one get if for certain dates or a period of time?
The visa will specify a date on or before which you must enter Australia. Once you enter, you may remain for up to 72 hours (clauses 771.611 and 771.511).
- Is it possible to change the dates or there is a need to issue a new one in case the flight dates are changed?
Once a visa has been granted, it cannot be varied. If your flight times have changed, you will need to apply for a new visa.