The Tax Administration's online OmaVero service is to be offline for several days due to a systems upgrade.
The outage starts at 3:30pm on Thursday and ends at midday on Monday, according to the agency, during which time users won't be able to log in to pay tax bills, change tax cards or conduct other business.
Even employees won't be able to access data or systems, and therefore even those attending service points won't be able to get a new tax card or any other tax services.
Users are advised to get any pressing tax matters sorted before the break in service starts. A limited phone service will be available on Friday, offering general advice but no specific help on individual tax matters.
"The operational logic will change a little," said Joona Jarva, the OmaVero product owner at the Tax Administration.
The new service is expected to function better on smartphones and tablets.