A wall of large sandbags filled with stones was placed to close the hole in the flood barriers on Monday.
But powerful flood currents breached the barriers yesterday.
Authorities and workers had to rebuild a sandbag dyke 200m away.
Up to 2,000 gabions _ rectangular wire mesh baskets filled with rock _ were brought in to reinforce the sandbag wall and were installed overnight.
Pumps were used to drain floodwater out of the downtown area, which affected people living outside the sandbag wall, with water rising to more than 2m deep.
Natural Resources and Environment Minister Preecha Rengsomboonsuk said floodwater would be pumped out of the municipality as soon as the gabions are in place. He said Sukhothai municipality would be safe and dry in a few days.
Overflow from the Yom River hit Sukhothai municipality on Monday, breaking through eroded levees beneath flood walls to swamp parts of the city.
The flood occurred because the bases on which flood walls had been built unexpectedly broke.
The surging water made many roads in municipal areas of Sukhothai's Muang district impassable, with some of them under up to 1m of water.
Sukhothai governor Chakkrin Plienwong said a temporary shelter for up to 1,000 people was set up at the provincial physical education college.
State agencies sent six-wheeled lorries and flat-bottomed boats to transport residents.
Sukhothai public healthcare chief Chinoros Leesawas said mobile medical units were deployed to help people suffering from illnesses during the flooding.
Science and Technology Minister Plodprasop Suraswadi, who heads the Water and Flood Management Commission, yesterday apologised to Sukhothai residents.
He said the flood levees designed to prevent overflow from the Yom River were built in 2001 and the structure may have eroded through years of wear.
It was difficult to detect damage in the levee bases, he said.
However, he expected flood levels to recede in the next few days and said downstream provinces were not likely to be affected.
Meanwhile, six districts in Ayutthaya have been flooded, provincial governor Witthaya Phiewphong said yesterday.
They are Bang Ban, Phra Nakhon Si Ayutthaya, Sena, Phak Hai, Bang Pa-in and Bang Sai districts. Mr Witthaya said he had ordered district chiefs and local administrations to keep watch and provide assistance for affected communities as needed.
Koh Muang _ a large community on about 5,000-6,000 rai of land where the province's central business district, major hospitals and historical sites are all located _ was not flooded.
Royal Irrigation Department director-general Lertviroj Kowatthana said yesterday the release of water from the Chao Phraya Dam in Chai Nat at a rate of 1,800 cubic metres per second had caused the Chao Phraya River and the Noi River to overflow and inundate riverside communities, but it would not flood farmland in irrigation areas.
He expected the water levels would stabilise and go down if there was no further rainfall in the next few days.
Train services to Chiang Mai resumed at 5.30pm yesterday after staff repaired a damaged track in Lamphun, the State Railway of Thailand said.
Heavy rains and a landslide caused a sinkhole underneath the railway between Khun Tan and Tha Chumpoo stations in Lamphun, which on Saturday prompted the SRT to suspend services between Lampang and Chiang Mai.
In Uthai Thani, run-off from Mae Wong National Park in Nakhon Sawan swept into the Tak Daed River in Sawang Arom, Thap Than and Muang districts yesterday morning, with several communities and farmland under 50cm of water.
Bandit Inta, director of the Phitsanulok irrigation project, said the run-off from Sukhothai was expected to reach Bang Rakam district in the next few days. He told people in riverside communities to prepare to move to high ground. Deputy Prime Minister Kittiratt Na-Ranong said yesterday he was confident flood prevention measures devised by the government would protect six industrial estates in Ayutthaya from a repeat of last year's flood crisis. The measures include construction of concrete flood barriers around the estates.
However, Mr Kittiratt said the government could not afford to be complacent and it would monitor the flood situation closely.
He added that the government would be ready to compensate farmers whose farmland is affected by the deluge.
Meanwhile, His Majesty the King yesterday had another 1,500 flood relief packages delivered to flood victims in Sukhothai through the Rajaprajanugroh Foundation.