As part of this challenge we are provided with the following description: "Right at the heart of the nation, no piece of the bush inside the circle remains untouched by us" and a photo said to be retrieved from the website in the previous challenge.
Analysing the features of the photo, we can see a dirt path, a semicircle of stones that appear to be a seat/bench or feature of some description, to the right of the photo something that resembles an information board and in the direct background some street lights. The combination of the caption and the street lamps highlights that whilst the photography was taken in a semi-dense bush area, there may be an adjacent street quite close by.
The phrase "heart of the nation" alone could refer to numerous things such as charities, however in the context of the challenge (geolocation) it is used to refer to a location. The phrase 'heart of the nation' could be the "literal" middle of Australia, such as somewhere near the city of Alice Springs in the Northen Territory. However, when comparing this photo provided to photos of Alice Springs found through common search engines, it is evident that the green, lush, semi-dense bush is not a common sight you'd find in the middle of the outback where the canopy might be green, but the bush is often scarce, not very dense dried, cured, brown colours. After ruling out Alice Springs, that leaves a second potential location, the nation's capital - Canberra, Australian Capital Territory (ACT). This isn't the only hint to Canberra being the location in question, the phrase 'a piece of the bush remains untouched', is a tribute to Canberra's well known colloquial nickname "The bush capital". The bush capital is well renowned for its urban forest through its preservation of greenery, streets interwoven with nature as well as being surrounded by bushland (See: https://en.wikipedia.org/wiki/Canberra).
There are many, many references to Canberra being called the bush capital, any search engine should return results for Canberra when searching a combination of terms including but not limited to "Heart of the nation" "bush" "Australia", etc. For example, one article found using a search engine (See: https://www.canberratimes.com.au/story/6035384/the-bush-capital-canberras-new-number-plate-slogan/) notes the usage of the phrase "The bush capital" on ACT number plates. Similarly, the term 'Heart of the nation" is feature in the article where it was actually considered as a number plate slogan for the ACT!
With this in mind, using a online mapping service such as Google Maps and searching Canberra or the ACT, we get a birds eye view of the capital. This is where the 'heart' reference particularly comes to play, where it refers to the part of the nation that keeps it alive, that is the federal government (and federal agencies). To narrow down the location further, the challenge was lacking a solid reference until I addded "inside the circle" to the caption. Canberra is also well known for its geometrically shaped roads (hexagons, circles, triangles, etc), however the 'circle' here is a direct reference to the road type (like street, drive) and shape of the two roads - the State and Capital circles that surround Parliament House (where the Federal Government meet - see previous hint about heart of nation). From here, the competitors need to use their analysis and deduction skills from the photo (such as identifying the street lamps and stone semi-circle) along with online mapping tools such as birds eye/satellite view, Google Street View, Google Photo Sphere to identify any of the key features in the photo.
There are only a few of the stone features within the State Circle area, some are visible from satellite view where as others are not, some also have Google Spheres at them or are visible from street view. When a stone feature is identified through any means, they will be able to compare the surroundings to the photo provided. Some techiques involve using the street lamps and their alignment, tree line or angle that the photo was taken at (with reference to other objects) for example to narrow down the location further. The location of the semi-circle stone in question has a google sphere a few meters away from where the provided photo was taken, once the user has found this, they just have to submit the coordinates to three decimal places. Whilst Google Maps provides coordinates to 6 decimal points accuracy (-35.306943, 149.120724). We chose three decimal places to provide a margin of error if the user's coordinates were slightly off. It should be noted that because of the three decimal places, the coordinates submitted will actually point to the middle of the State Circle road.
The flag is DUCTF{-35.306,149.120}