The deluge was caused by breaches in the floodwall and the overflowing Yom River north of the flood barrier.
The water flowed into Jarodwithee Road and flooded some residential areas. The floodwater was about 50cm high on average.
Workers yesterday tried to repair the breaches and pump water out of the area.
Mayor Pichet Thaikla said the water leaked under the barrier before flowing into the municipal area.
However, the situation was under control and did not call for any evacuations, he said.
Natural Resources and Environment Minister Preecha Rengsomboonsuk said a top priority was to monitor and maintain the water level in the Yom River.
Mr Preecha yesterday inspected the flood barrier along the river.
He said the situation was likely to return to normal soon because the water was flowing into the municipality at 400 cubic metres per minute, down from 1,200 cubic metres of water per minute.
He said authorities would accelerate the drainage of water as soon as water levels in the river receded.
"The situation should improve soon. It is not that serious because the volume of water in the river is half of last year," he said.
Authorities in Phitsanulok yesterday advised residents in three districts _ Phrom Phiram, Muang and Bang Rakam _ to brace themselves for rising river water.
The floodwater level in the three districts, which are already submerged, was likely to increase by another 50-70cm. In some low-lying areas the water level might rise to over 1 metre.
Governor Chairoj Meedaeng said the second mass of water was expected to reach the province in a day or two.
Residents living along the riverbank have been strongly urged to move their belongings to high ground.
The three districts were flooded after Sukhothai sped up drainage of water in the municipality through the Yom River.
Her Majesty the Queen yesterday donated 931 sets of flood relief supplies to residents in Phrom Phiram and Bang Rakam districts.
About 22,500 rai of farmland in the two districts and about 1,000 households were inundated.
Meanwhile, Science and Technology Minister Plodprasop Surasawadi yesterday expressed confidence that a flood disaster would be averted this year.
He said the government faced two challenges in flood management _ the water, and the people's fear of a repetition of last year's flooding.
"There is nothing we can't handle. We have the experience. I don't think there's anything we can't manage," he added.
He spoke during a video-conference with governors of 36 provinces following the breaches in the floodwall in Sukhothai. Prime Minister Yingluck Shinawatra had ordered Mr Plodprasop and Deputy Prime Minister Yongyuth Wichaidit to hold an urgent meeting to prepare for the run-off from the North.
Mr Plodprasop said the prime minister wants state officials put on stand-by and to help evacuate people before the flood hits.
"This year there won't be people walking through floodwater. The state officials must be there in the area or they will be in trouble," he said.
Mr Yongyuth yesterday called on the media not to sensationalise the flooding. He said the situation in Sukhothai is not as serious as seen in the news.
Meanwhile, Royol Jitrdon, chairman of the sub-committee monitoring floods, yesterday said the runoff from the North should not pose any threat to Bangkok.
He said that there were only 300 million cubic metres of water this year, compared with 1.2-1.4 billion cubic metres of water last year.
Below are scenes by Bangkok Post and Post Today photographers of the Sukhothai flood on Sunday.