a shout
Buying a round of drinks is a custom in Australia, as in many corners of the world. It is generally expected in a pub that when you arrive and make your first trip to the bar that you will offer to buy a drink for others you are drinking with. Similarly this will likely be done to you when someone else joins the group. This is called a shout, and incurs an obligation that you will generally return the favour in a following round, and that also you will generally maintain the same drinking pace as your associates in the round throughout the evening. If someone in the same round as you has an empty glass, who is ahead of you in drinks bought, you should declare that it is your shout, and make your way to the bar. If someone offers to buy you a drink, but does not offer to buy for the person who already has bought you a drink, you should say you are already in a shout, and decline. If they buy you and the people in your round a drink, they have joined the shout. Its generally not polite to switch between shouts during an evening. It you are in a large shout, and you decline a drink, you still have to buy a drink for the round when it comes to your turn. You are well advised if you wish to skip a round, to do so on your shout. It is generally poorly received to buy a round, and then to refuse a drink when one is purchased for you. Often the drink will just be bought for you without even asking. Don't be surprised if someone who bought you a drink earlier in the night, later says that it is your shout. Not joining a shout can be awkward in some groups. The best way is to say you are driving, and you will buy your own drinks. This is also an acceptable way to drop out after one round, when the score is even.
legal aspects
The legal drinking age throughout Australia is 18 years. It is illegal either to purchase alcohol for yourself if you are under 18 years of age. It is illegal to purchase alcohol on behalf of someone who is under 18 years of age. The only legally acceptable proof-of-age is an Australian drivers licence, state-issued proof-of-age card or a passport, and it would be wise to carry one if you want to purchase alcohol or tobacco and look under 25. It is illegal to go into a gambling area of a pub or club when under 18.
Often there is a lounge, restaurant or bistro area in a pub or club that permits under-age people provided they are accompanied by a responsible adult over 18 and don't approach the bar or wander around. Some city pubs even have video games and playgrounds for children. Some country pubs have large open areas out in the back where kids can run and play.
In general, you can take alcohol say a bottle of wine or beer to consume at a park or beach. Alcohol consumption is banned in some public places as 'street drinking'. These are often indicated by signs and is particularly the case in parks and footpaths where public drunkenness has been a problem. However, if you are a family with your picnic basket and blanket out at lunchtime with a bottle of wine, you are unlikely to encounter any problems.
Alcohol can be purchased for consumption on premises only in licensed venues: pubs, clubs and many restaurants. You can purchase alcohol for private consumption in bottle shops, which are separate stores selling bottled alcohol. In some but not all states you can buy alcohol in supermarkets. In those states where you can't, bottle shops and major supermarkets are often found in very close proximity. Although licensing laws and hours vary from state to state, and individual stores have different trading hours, as a rule of thumb, alcohol is generally available in towns to take-away seven days a week, between the hours of 8AM and 11PM, from bottle shops, supermarkets, licensed grocers/milk-bars and pubs. Outside of these hours though, it is almost impossible to buy alcohol to take home; unless you're in the middle of Sydney or Melbourne, so if you're planning on a party at home; it's a good idea to stock up and check on the local trading hours so you don't run out at 00:30 with no opportunity to buy more. Alcohol is not available at petrol stations or 24-hour convenience stores anywhere in Australia.
Public drunkenness varies in acceptability. You will certainly find a great deal of it in close proximity to pubs and clubs at night time but much less so during the day. Public drunkenness is an offence but you would only likely ever be picked up by the police if you were causing a nuisance. You may spend the night sobering up in a holding cell or be charged.
Driving while affected by alcohol is both stigmatized and policed by random breath testing police patrols in Australia, as well as being inherently dangerous. Drink driving is a very serious offence in Australia, punishable by a range of mechanisms including loss of license. The acceptable maximum blood alcohol concentration is 0.05% in all states, often lower or not allowed for operators of heavy vehicles and young or novice drivers. Police officers are also empowered to randomly test drivers for the recent use of prohibited drugs. The operation of a motor vehicle while under the influence of prohibited drugs or alcohol will always result in arrest and a required court appearance many weeks from the date of arrest and it can comprehensively disrupt travel plans. Random breath testing is common early Saturday and Sunday mornings, and many people are caught the morning after.