A massive clean-up is ahead for parts of Otago and Canterbury after a storm that flooded homes and farms, ripped up roads, cut off towns and sliced a bridge in half.

Many roads are still closed and as river levels and flood waters drop, people are waiting to see the full extent of the damage.

In Omarama, dozens of people had to flee the Top 10 Holiday Park and owner Irwin Beiboer said many of the units were still underwater.

"There's too much water flowing through there (to check the remaining units)," he said.

The bridge into Lake Ōhau has been completely washed out. (Source: 1News)

READ MORE: Flooding, slip cut off Aoraki/Mount Cook, Lake Ōhau villages

"We think some units have been hit the hardest because they're standing at the lowest place.

"The units we have checked, the carpet is soggy, the showers, the toilets, everything is just muddy and wet."

Flooding at Top 10 Holiday Parks in Omarama. (Source: 1News)

Further south in Otematata, where several homes were flooded, locals are calling the storm the biggest in nearly half a century,

Volunteer firefighter Richard Jopson said water was still flowing down some streets last night despite the rain easing.

"There's still a bit of water around," he said.

"We evacuated about three homes to be fair and those people are happy now.

"We'll just continue the clean-up [Wednesday] and see what the day brings."

The swollen North Branch Ashburton River (Source: 1News)

The only bridge into Lake Ōhau, near Twizel, has been washed out, leaving more than 100 holidaymakers at the local lodge stranded.

Fixing it is a top priority for Waitaki District Council.

Waitaki District mayor Gary Kircher is working out how to make sure people don't run out of the essentials.

"They're okay for the time being but it is just a matter of making sure they've got food supplies and all the bits that they need to look after the people that are in there," he said.

"We're still working on the logistics around making sure those people are properly looked after."

READ MORE: Photos: Slips, flooding, high rivers hit parts of South Island

Mount Cook Village has also been cut off after being hammered in the storm.

More than 400mm of rain fell in 24 hours at the village.

A big slip closed SH80 leaving local man Charlie Hobbs unable to get home on Tuesday.

"I'm hoping they'll be able to shift that debris which is about 80 to 100 metres wide covering the road," he said.

Waka Kotahi expected to be able to tell people by midday when the road will re-open.

Stock near the Ashburton River (Source: 1News)

In Mid Canterbury, the Ashburton River burst its banks and flooded neighbouring farmland.

Rhys Roberts has farms south of the river in Mount Somers and Westerfield and said both parts of both properties were underwater.

He said the next few days would be assessing the scale of the damage.

"One of the farms is a dairy farm that's got cows calving down in the next five days," he said.

"For us it's quite important to get that farm back up on its feet so there'll be a lot of effort go into this farm.

"We're hoping it hasn't caused much scouring but we'll be re-assessing that."

Ashburton Mayor Neil Brown said the river peaked on Tuesday night without causing any more flooding and he was confident the worst was over.