Earthmovers have been called in to urgently excavate a new opening for the mouth of the Rakaia River to try to prevent flooding at a settlement following heavy rain.
Overnight on Friday 156mm of rain fell in the river's catchment and a further 100mm was forecast for Saturday.
Regional council staff carved out the new river mouth on Friday.
Selwyn District Council's corporate services manager Douglas Marshall said the previous opening had left Rakaia Huts residents near the river mouth vulnerable to flood and storm surges.
Mr Marshall says high river flows will be diverted through the new opening, further to the south.
People should take care around both the Rakaia and Waimakariri Rivers as water levels are likely to rise quickly.
Mr Marshall says residents should secure their home, pets and livestock, listen to the radio for updated information and inform neighbours if they need to leave.