Not to mention that there are many other already public & private records that already can provide information on wealth (company ownership records, house ownership records, etc...)
All incoming and outgoing correspondence is kept on a public record and can be requested. So if you send in a request per e-mail or post, it can be accessed. But you can still can in anonymously or walk in to their office and request it physically, without having to identify yourself.
Source: Used to work as a journalist in Sweden, dealt a lot with requesting information from governmental and military organizations.
Everyones's reported income, address, date of birth, family and a whole lot more information is publicly available on request. If you have particular reasons (e.g. threat of violence), you can request to have your identity hidden and exempt from public disclosure.
Personally, I'm not sure I'm so super happy with this system, but it's very much in tune with our culture where being in a significantly better position than the average is seen as suspect.
A perhaps internationally unusual thing this creates is that entire police investigations, including interview transcripts, are usually available to download as soon as court proceedings begin.
As a side note, this transparency helps expose potential corruption in handling evidence, awarding defense contracts, etc.