Police said the right-wing extremist had received 200 to 300 letters since he was jailed after the deadly killing spree.
“He’s having the letters delivered to him once we’re gone through them,” police lawyer Christian Hatlo told broadcaster TV2.
Breivik has received two batches of letters since the recent loosening of restrictions on his contact with the outside world.
The letters include a number of bills, TV2 reports, but the broadcaster also learned that the perpetrator of the July massacre had begun reading through the many love letters and hate mail he has received from Norway and abroad.
Postal and visitation restrictions mean the police are authorized to examine his letters in advance and remove any that may be deemed harmful to the ongoing investigation.
But while much of the correspondence is of a threatening nature, Brevik also has his share of romantic admirers, as well as would-be pen friends offering religious redemption, TV2 reports.
Starting on Monday, Breivik will also be permitted to watch television, listen to the radio, and read newspapers.
Breivik is expected to be sentenced to psychiatric care after prosecutors last week declared him criminally insane. A report submitted by expert psychologists found that Breivik suffered from paranoid schizophrenia.
The controversial findings have divided opinion. Writing in newspaper Dagens Nyheter on Wednesday, Swedish professor of psychiatry Johan Cullberg described the report as “objectionable, unprofessional, and lacking in imagination.”
Breivik’s dual attacks primarily targeted Norway’s Labour Party, a group he said promoted the multiculturalism he despised.
A car bomb outside government buildings in Oslo killed eight people before Breivik gunned down a further 69 on the nearby island of Utøya, where members of the Labour Party’s youth wing were attending a summer camp.