The Revenue's online system was overwhelmed last week, with taxpayers trying to file returns ahead of the 30 September deadline, and thousands of people were locked out of the system.
"It sets back the cause of e-government if systems are brought online that result in a poor user experience," said Richard Allan, the Liberal Democrats' IT spokesman.
After "round-the-clock" efforts by the Revenue and its IT services provider EDS, the service was brought up to speed for last weekend, when a record 17,000 people filed returns electronically.
"Since the changes have been made customers have been able to freely access SA Online once more but we will continue to monitor the situation closely," a Revenue spokesman said.
"The root cause of the problem was the Network look-up tables - a communication link on SA Online between its Web hosting environment and its database."
Changes were made to the network look-up tables' configuration but this then required the servers to be rebooted, further disrupting the service.
Mike Thompson, principal research analyst at Butler Group, was not impressed. "Why was this not discovered last year?" he asked. "Didn't they do capacity testing?"
Thompson said the SA Online service faced steep peaks in demand around the 30 September and end-of-year tax deadlines.
"In a commercial environment, the organisation would build in redundancy to cope with the spikes in demand because it did not want to lose business. The Inland Revenue has got the business anyway," he said.
Chas Roy-Chowdhury, head of taxation at the Association of Chartered Certified Accountants, described the continuing problems with SA Online as "a fiasco". The US government has a quarter of taxpayers filing online with few systems problems, despite charging for the service, he said.
"In the UK, less than 2% of taxpayers file online. If the system cannot cope with such low volumes, how is the Inland Revenue going to convince others to use the system?" he asked.