Texas Governor Greg Abbott has likened the ferocity of flash flooding that killed at least three people to a tsunami, and authorities said a dam had given way in a state park.

Abbott declared states of disaster in 24 counties and flew over the area south of Austin to assess the damage caused by tornadoes, heavy rainfall, thunderstorms and flooding that forced evacuations and rooftop rescues and left thousands of residents without electricity.

"This is the biggest flood this area of Texas has ever seen," Abbott said. "It is absolutely massive - the relentless tsunami-type power of this wave of water."