I've not ever built an outside-Aviary, but my 8 English/American Budgie Hybrids live in a walk-in Aviary that I built in my house, and they come out each day for 3-4 hours in the house with me and to fly around. Mine is built-into a wall in the walk-out floor of my house (3-story split-level house with the walk-out level being under the house and opens to garage)...This is a lot different than building one outside, but there are some things you need to make sure of BEFORE you do this...
First of all, as Sailboat already mentioned, there MUST BE an area of any outdoor Aviary that is covered for the safety and the health of your birds. And this covered-area of the Aviary is where you should put ALL food and water dishes, so that it doesn't get wet, no rain-water gets into their fresh drinking water, etc. You simply cannot allow their food to get wet because it immediately starts growing Bacteria, Fungi (yeast), etc. Plus it will get dirt and all kinds of other things in it if it's not under a covered area...Most-importantly though is ensuring that you have a large enough section of the Aviary covered that ALL BIRDS inside of the Aviary can get shelter/coverage at the same time if need-be. I know you live in Australia so you don't have to worry about the cold-weather, and that's great, and it's great that you are allowing your Budgies to get fresh air and Sunshine, but that doesn't at all mean that you don't have to have a large section of your Aviary that is covered...
***Many people here in the US who live in States that are warm/hot all year round and have no winter, such as Florida, Georgia, Alabama, Texas, Southern California, etc. keep their pet birds/parrots in outdoor-Aviaries full-time, and are living in very similar climates as you are in Australia (specifically areas of Florida). i actually know a few people who live in the Fort Lauderdale/Deerfield Beach/Miami area who have hand-raised, tame Macaws and one who has an Umbrella Cockatoo and who built outdoor-Aviaries for their birds, and that's where they live year-round (though they bring them inside of their houses often and usually they sleep in a cage inside of their houses because of the fear of predatory animals/birds getting to them at night, such as Raccoons, Opossums, Owls, Snakes, Lizards, etc. which are nocturnal and get after birds all the time)...What all of their Aviaries have in-common is that they all have a very large section that is covered; I have seen 2 of these in-person, both for Macaws, and they are actually both COmPLETELY COVERED ON TOP...Their Macaws stay inside of these outdoor Aviaries during the daytime while they're at work, and then they bring them inside when they get home, and put them back in the Aviaries in the morning before they leave. Their birds still get plenty of sunshine, but they never get rained on and they can always get out of the sun totally whenever they want to, which is absolutely mandatory for an outdoor aviary, because if your Budgies don't always have the capability to get out of the Sun completely at any time they want to/need to, you are going to end-up with Budgies who are suffering from Heat-Exhaustion, Heat-Stroke, and death. Plus, they are going to be eating food and drinking water that is contaminated and unhealthy. So if you don't already have a large area of the Aviary that is completely covered with a "Roof" of some kind and where you need to locate all of the food and water dishes/containers, then you need to fix this immediately (it doesn't sound like your Budgies have any covered area of their Aviary at all, if they do then I apologize, and that's where you need to put their food and water, all of it)...
****The other thing that you need to make sure of is that the "Mesh" you used, if some kind of Fabric/Plastic/Resin material, is something that not only your Budgies cannot chew through, but more-likely and more-importantly that no wild, predatory animals and/or birds can chew through it...AND if it's some type of Metal mesh or "Hardware Cloth", that it's 100% Stainless rather than Galvonized, because not only will it rust if it's not Stainless, but if your Budgies chew on it (and they will), it will cause them to suffer from Heavy-Metal Poisoning, which is fatal without going through a long, expensive Chelation-Therapy...