The South African Revenue Service says it will not extend next week's tax filing deadline regardless of its employees being on strike.
Deadline for manual returns submissions is September 18, while electronic submissions must be in on or by November 20.
Sars spokesperson Adrian Lackay said: "We are not extending the date. The deadline still stands."
Sars has initially offered workers a 7percent increase, while unions demand 13percent. The employer then made a revised offer last week of between 9 and 11percent on a sliding scale.
The National Education, Health and Allied Workers Union and the Public Servants Association are demanding a 12,8 percent increase across the board.
Lackay said Sars would ensure that offices across the country remain functional and accessible to taxpayers.
PSA spokesperson Manie de Clercq said: "Our members have rejected the employer's counter offer."
Nehawu has also called on its members to withdraw labour until their demands are met.