Minister for the Environment Phil Hogan has said he expects up to 75% of all septic tank owners to have registered by the deadline tonight.
Mr Hogan said 68% of septic tank owners have already registered.
The fee is €50 and registration can be done at www.protectourwater.ie, by post or at local authority offices.
The minister warned that those who fail to register in time would be breaking the law.
In addition, they would not be entitled to financial assistance if they have a problem with their tank in the future.
It is estimated that there are between 450,000 and 500,000 septic tanks in the country.
Any extension to the deadline has been ruled out.