It's actually a fairly long tradition, it's only been online for the previous decade or so. I'ts easy to access, it's just that for the last few years, you can also see if someone has checked your taxes and who they are.
I think it's a Nordic thing. Tax records (including recorded income) are also public in Finland, and apparently Sweden has something similar. [1]
In case of Finland, the current legislation that makes tax information public was originally introduced in 1999 but I can't remember whether the records were also public (based on some other regulation) prior to that or not. In any case, it's not that recent. The Reuters article says Norway has had public tax information since 1863, but I don't personally know anything more about that.
AFAIK anybody's tax records are basically a phone call away. You can't just google for the information, though. I don't know how it works in Norway. (Edit: but apparently the sibling replies do.)
To give the Swedish story. In general all documents, decisions, etc. handled by a public agency are by default public (i.e you can call/email the agency and ask for them).
So when the tax agency makes a decision on your taxes that becomes public, i.e we can see what taxable income you have. One way this is used is by newspapers to look into the income of politicians (and other famous people..).
The right of public information is taken quite seriously by the courts (and should be taken more seriously by agencies that really like to classify the information as secret, which you then have to go to court to challenge). For example an organisation I'm associated with was able to get the cookie data from the Swedish Chief of Police which the courts determined was public information (although they were allowed to mask some information).
Is it a somewhat new regulation? Is it easy to access the information?