The chances of more cattle slipping from riverbanks and dying were high, he said.
"On the bank from the river onto the berm, there are holes and mud, it's not in good condition. It's hard enough for me, let alone a cattle beast."
A plan change was due to come into force in 2020 excluding cattle from the Tukituki River.
The rule would not apply to sheep, which Mr Hawke said he has also seen grazing beside local waterways.
The Hawke's Bay Regional Council chairman, Rex Graham, said he was surprised to hear of stock in local rivers.
"Cattle grazing in rivers is just a no-no," he said.
Council chief executive James Palmer said farmers should not graze stock by waterways.
"Council's position is that stock should not have access."
A plan change banning cattle grazing by the Tukituki River was due to be implemented in 2020.
"The plan already exists ... there are various implementation dates."
The council was working with farmers to prevent stock being in rivers, Mr Palmer said.
"We do have some farmers in the catchment who are showing unwillingness."
Mr Hawke said he was told stock were allowed to graze on riverbanks because it was good for flood management.
Mr Palmer said this was not the case.
"It's possible confusion may arise because in some parts of our flood management scheme, we do have cattle grazing on stopbanks."
This was under review, Mr Palmer said, and it was a council policy objective to stop the practice of stock grazing by waterways.