John McAlister, e-tax national director, said, said the lodgement systems were unable to cope with a last minute rush by self-preparers trying to lodge their returns ahead of tonight's midnight deadline.
ATO deputy commissioner for personal tax, Raelene Vivian, said as many as 4,500 users across Australia tried to access the system via the ATO's Web site concurrently between 11pm Sunday and 12am Monday.
"We've found we've got a lot of concurrent users - people leaving it to the last minute to lodge -- so we've found that people are getting some slower responses or an error message saying that the service is unavailable," said Vivian.
However, it's not clear whether the service slowed down or suffered a system failure as demands on the ATO servers peaked.
"I think it would be too early to say that at the moment. We're doing all we can to restore acceptable user experience. We'll look at the problem after that," said McAlister.
Vivian said that while the ATO did have contingency plans to deal with expected surges in demand in place it would be taking a closer look at this year's lodgement patterns in preparation for next year.
The ATO has recorded a 22 percent jump in the number of tax returns processed through its online lodgement service to date this financial year. Last year 834,000 returns were lodged online, but so far this year the ATO has received 953,000 and its forecasting that the number will reach 1 million before the lodgement season ends.
The ATO has extended deadline for individuals to lodge personal tax returns without penalty to midnight Wednesday.
It has also encouraged users experiencing problems with the lodgement system to contact its support desk on 1300 130 017.