hello.. I second Genie47 on his suggestion of the great southern route (aka Melb-->GOR --> Grampians --> Melb)
1. The price of petrol in Australia (Will rent a car from Hertz)
Petrol Prices varies from region to region.. regular 91 unleaded ranges from 1.50 to 1.35.. typically prices are cheaper in the city area and gets more expensive as you move further from Melbourne... my suggestion is once u see a cheap petrol station, just fill it up even if the car is half full as you never know if there will be a cheaper station down the road.
2. Accommodation (Economic)
Again it ranges depending on where you wanna stay. for touristy places like GOR, it will be more expensive, if you got a car, maybe can stay some place out and drive into town when necessary. expect to pay per room between AU$150 - 200/night
3. Weather condition (I think it is cold)
The key is wear multiple layers. When i was there 2 weeks ago, the weather can change from 14 deg one day to 32 deg the next...so bring a waterproof jacket, long sleeve tee etc ....
4. One day per location can do how many places of visit
again depends, i suggest staying 2-3 nights at GOR, another 2-3 nights at Grampians (if you are a avid hiker)
5. My initial plan is to visit Melbourne City for one day tour then later starts the touring for 7 days
You can go for the wine tour at Yarra or penguin at Philip I....up to you..7 days along the southern route is quite relaxing and nice pace...
8. SIM card for data plan
i got the Optus card (AU$2/day plan)... but the reception along GOR is intermittent at best.. along the touristy area, its fine, can check FB, upload pictures to irritate your friends, but once out of the town area, i get no signal unfortunately...
9. Food and Drink (Economic)
if eating out every meal, expect to pay between $10 - 25 per pax, or u can cook your own which will be cheaper. No Mcd or KFC along GOR until Warnaambool.
10. Expenses allocation for the whole trip (inclusive air tickets x 2 pax): $3.5k
again depends on how much u paid for your airfare... but on average of $700/pax for airfare, leaving you with 2k expenses for a 8 days trip with car rental might be pushing it, but again, if you can tahan backpackers lodge and simple meals, why not?
Thanks for your advise. I think my wife and I are not fussy for the food and accommodation. Most importantly, is the rental car must be able to complete the whole journey w/o giving us problem.
I think I will go with Hertz as I saw a lot of forumers recommended that.