To answer your question you need to decide on a few important things. First off, what kind of accommodations are you two looking to have? Do you want to be staying in hostels, or hotels? If you want hotels, are small places fine, or do you need the big four-star establishments? For food, do you want to do it on a budget by picnicking a fair bit, or do you want to eat out for each meal?
Just to give you an idea, my girlfriend and I backpacked through similar countries as you are planning on, and we stayed primarily in hostels and ate on the cheap, with two self-catered and one restaurant meal per day. To do this, we had budgeted $35/person/night for accommodation and $25/person/day for food (all funds Canadian dollars, but it is virtually the same rate as Australian dollars, so I think these numbers will be applicable to you).
At the end of our trip we evaluated our budget. We were sadly over-budget on accommodations because we stayed in some expensive places (London, Paris, Venice) and a few times decided that we wanted to opt for a private room when it still fit our budget, rather than saving money on a shared hostel room in the cheaper places. Fortunately, we actually came in well under-budget for food, and that was making sure that we ate at least one nice meal per day (though very little alcohol - you will need to budget more if you wish to consume much of that).
In total, we spent $5500 per person for a full month in England, France, Italy, Germany, Switzerland, Austria, and the Czech Republic. That included airfare (which is expensive from Canada), rail pass, food, accommodation, admission fees, and a few nice souvenirs. Obviously the cost of your trip will vary based on what kind of accommodation and eating you wish to have, and which sites you wish to be seeing, but I think the information I have presented above will give you a starting point.
Just a couple of tips. First off, the more research you do in advance, the more time and money it will save you while you are in Europe. If you already know what sites exist in a certain city, you save time trying to find a tourist information office just to know what to do. Also, if you knew to book in advance, you will save money on most travel options once you are in Europe.
More specifically, if you are going to stay in London or Paris, make sure you book your accommodations well in advance as they book up - especially the good-quality yet still inexpensive places. If you will be going to Venice, splurge a little bit extra to make sure you stay in Venice itself rather than on the mainland. Here we gave up a well-priced hostel in Mestre (the mainland) for a fantastic B&B right in Venice. It was expensive, but Venice late at night and early in the morning is a completely different place, and well worth the extra expense.
If you have more specific questions I would be happy to help out. Feel free to go into my profile and email me any questions directly.
Have a great trip, and happy planning!