I see a lot of people doing this. I believe this is not safe, isn't it?
Where can I do this?
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Kangaroos are wild animals. While you will come across them if you are driving in rural areas, do not approach them. While most will simply run away, some may enter a more defensive posture especially if there are young (joeys) nearby. A kick from an adult can kill a human (http://www.amazingaustralia.com.au/animals/kangaroo_attacks.htm).
Kangaroos can be mean vicious creatures. When I was growing up as a kid in Australia, we were cautioned away from getting anywhere near them.
The best and safest way to get a selfie with a kangaroo that isn't a cardboard cutout is to go to a zoo that has a petting zoo component. Instead of kangaroos, they often pick the smaller/friendlier cousin, the wallaby (http://animals.howstuffworks.com/mammals/wallaby-vs-kangaroo.htm).
This is just one of many petting zoos that have wallabies: http://www.featherdale.com.au/#=
It depends on how close you intend on getting for your selfies. If you insist on approaching closely, I would only do so in a controlled environment. If you're willing to have your selfie with the kangaroos a dozen or more metres in the background, you'll be fine.
As to where you're able to do so, just about every major town or tourist location has some location where you can get close to kangaroos - patting or hand feeding, either of which would allow selfie opportunities. Some examples include:
Searching specifically for whereever you may be visiting would almost undoubtedly find somewhere, as all international airports and ports would have somewhere within an hour or less.
Of course, you could just go golfing.
Mareeba Golf Course. Credit: Sweetwater Lodge
Yamba Golf Course. Credit: Shelley Street Beach House
Anglesea Golf Course. Credit: Anglesea Golf Club
The Cairns Tropical Zoo has a Kangaroo enclosure:
Spend time in our kangaroo area. Around 60 tame Eastern Grey kangaroos spend their day meeting visitors, eating from your hand and lazing in the heat of the day.
I can confirm that they will let you get relatively close if they are in the right mood, but not necessarily for the kind of "hugging" selfie you'd take with a human.
The Lone Pine Koala Sanctuary in Brisbane also has a kangaroo enclosure where you can hand-feed kangaroos, including (according to their website) Red Kangaroos and Eastern Grey Kangaroos. I visited many years ago and can confirm that they will eat out of your hand; however, selfies were not a thing then, and so I didn't try to take one.
If you are traveling to Melbourne, I recommend you visit Healesville Sanctuary, it's about an hour east of Melbourne, they have Kangaroo's they are quite used to people.
I went to the Featherdale Wildlife Park outside of Sydney when I was about 16. It was great and plenty of exotic(by my standards) animals running around that could be fed and touched. However, I don't recall any kangaroos that were free roaming in the park, I think they were all in enclosed spaces, but plenty of small wallabies running around like goons.