which city better

Hi Rishu,
Are you single, a couple or with children?
The with children families have big differences in costs between the cities due to state laws on school fees.
NSW is the most expensive in that regard,I think. eg.  decinternational.nsw.edu.au/study/schools/fees

If you are single, well it depends on your lifestyle needs. Sun and surf - perth and sydney. Sport, eating out and interesting night life - Melbourne.

Cost of living varies in all cities. Check out numbeo.com for variations in general living costs.
The cost of living within the cities varies greatly too. I find, the more expensive the rent/house prices in a surburb, the more expensive the groceries.
with my clients, i start with the end in mind. It is more important to be where you are happy and suits your lifestyle and then find an area within that city that suits your cash flow needs.

For civil engineering jobs Perth would be the best choice as West Australia has the most work opportunities of all the states.

For cost of living numbeo.com  as this site will give you a general idea of what to expect.

Taxes are high here in Australia as are rents.