A good place to start looking for drains is google maps and finding breaks in the grid of the suburbs. Long parks and strange curves in roads can be good indicators. Elevation can be used to find drains. Water must flow to the lowest point of a catchment area and this is where the big and walkable drains are.
Following rivers and creeks on google maps can also be a good way to find outfalls. Sometimes on maps there will be a short stub of where a river or creek abruptly ends. These are usually drains when in an urban environment. Google satellite view for most of melbourne also has 3D imagery which helps.
Now that you have found a drain, you may want to know how to get out. Some drains have another end you can just walk out of but most drains do not. As for finding a way in from outside, if you know the path of the drain you could pop random manholes and see if any of them connect up with anything bigger.
For gutterboxes, a green flag is if they are not actually part of a gutter but further back in the footpath. These gutterboxes usually lead to straight into the drain rather than a small side pipe as with most roadside boxes.
The most common manhole type for a drain is the humble gutterbox. A lot of newer suburbs have these plastic gutterboxes that are very light and easy to open, but they mostly come in concrete rectangular varieties.
Caution must be had when opening these as if they are rotated at all when lifted they may fall into the drain. This would be rather unfortunate if you were opening one from below.
There are many other ways in and out apart from gutterboxes. Circular manholes are one of the best ways in and out as they cant be dropped into the drain.
In victoria some sewer entrances are also circular manholes and labeled 'Sewer'. Majority of the time if it isnt labeled, it will be for a drain.
To open these from the bottom you just have to lift them up, but to lift them from above manhole keys will be required.
Another way into a drain is through a gatic. These aren't usually the best way into a drain as they are used for a heap of infrastructure and are usually very heavy.
Unlike other manholes and gutterboxes, gatics require a tool that is more hefty than a manhole key. With enough people and a gatic that is already loose, you could open one with a few manhole keys but gatic poppers are ideal.
Finding a way out from inside the drain can be tricky depending on what drain you do. Youd want to find a gutterbox exit as the lids are easy to lift and never on roads unlike regular manholes. Great stairway for example has close to no gutterboxes you can get out of so you would be looking for a manhole. Gutterboxes can be found down smaller sidepipes ~900mm that usually require a bit of a crawl to get to. Manholes are generally part of the main drain itself.
These are the most common shapes that drains will be in victoria. Round segments can come in any material but seeing round bluestone is pretty rare.
Square and rectangle segments are almost always concrete, rarely redbrick. Some square segments may have a pointed top/bottom.
Arch segments in drains can vary quite a lot. Some have very straight sides and an arched top, and some may be arched the whole way along.
Bluestone is somewhat common for the narrow arch variety. Arclight and 69er are good examples. Some bluestone arch segments may even be redbrick on top and pointed bluestone on the bottom. Concrete arch shapes are very common in the large underpass drains like Tenth and Mini 10th.
The Oviform, more commonly known as a balloon or mummy, is a rather unique shape that is found all over the older drains in victoria. Balloon shapes vary anywhere between narrow and wide. Punctual is host to some of the largest redbrick balloon in melbourne.
The narrow balloon shapes are almost always redbrick and the wider varient is almost always concrete like in Mummy.
