According to Metservice, 40mm of rain had fallen in Christchurch between midnight and 11am.
An orange heavy rain warning is in place for Canterbury north of Waipara, parts of Wairarapa and the Kaikoura coast and ranges.
A strong wind warning is in place for Banks Peninsula, Wellington, Wairarapa and the Kaikoura coast.
The Canterbury High Country, north of the Rangitata River, is also under a heavy snow watch with the forecaster warning snow could lower to 300 metres.
A rockfall closed SH6 near Murchison between the intersection with SH65 and Hinehaka Rd.
A slip has forced the closure of Dyers Pass Rd in Christchurch.
Motorists can travel as far as Summit Rd but cannot get beyond there. Contractors are on their way to try and clear the slip.
The Lewis and Porters passes were closed because of snow this morning but have since reopened.
SH80 Ben Ohau to Mt Cook reopened at about 9am after it was closed by a slip.
Meanwhile, the Timaru District Council declared a state of emergency for the Pleasant Pt Temuka Ward because of the risk of flooding.
Damage to a stop bank along the south side of the Opihi River between Butlers Rd and Kerrytown Rd means the flooding risk could be a threat to life and property.
The council says residents have been doorknocked and evacuated with the help of police.
However, the rain and flooding is starting to recede in Otago and Canterbury, although the village of Lake Ōhau could be cut off for days, RNZ reported.
Waka Kotahi NZTA has had a mammoth clean-up job to open roads again across the South Island today.
All road access to Lake Ōhau has been severed, at a time when the village is filled with visitors for the school holidays.
Floodwaters washed out the sole bridge providing access to Lake Ōhau, and the Waitaki District Council says it hopes to have access restored by the weekend.
Waitaki mayor Gary Kircher said Lake Ōhau was a very popular place at this time of year, with about 150 visitors staying at the Ōhau Lodge, plus others staying in Airbnbs.
It is also a village with no shops.
"The emergency management team are looking at how they can get some supplies into Lake Ōhau village mainly for those who are staying there longer than they were planning to and don't have enough food."
This morning, Ōhau Lodge owner Mike Neilson led 19 four-wheel drives across a paddock for people who needed to leave.
"We took a convoy of guests who had finished their stay here, and did a convoy and went across the farm paddocks. [I was there] to keep them in order and make sure they didn't wander off and not quite follow the right route and get lost and stuck."
Ōhau resident Hugh Spiers said locals tend to be self-sufficient, but he was still desperate for the bridge to reopen.
On Monday he was expecting a long-awaited delivery of Gib board needed to make repairs after a massive fire in the village in 2020.
"We're just on the rebuild programme at the moment after the devastating fires of 2020. We've had floods and fires, and we just don't need locusts. Hopefully that's the next thing that's not going to happen."
Omarama Top 10 Holiday park co-owner Irwin Beiboer said they were just starting to assess the extent of the damage, but at least 12 units had been damaged.
"[We] have the fire brigade here pumping the rest of the water away from us, as the water just doesn't go anywhere. The roads are all completely destroyed here at the park, units completely destroyed. There's a lot happening."
- Additional reporting RNZ
