Heavy rain has continued to fall across Banks Peninsula, keeping streams and rivers high and leaving floodwaters and hazards on a number of roads.

Christchurch City Council said there had been some easing in intensity, but conditions remain challenging in several areas.

State Highway 75 into Akaroa remains closed, with residents and visitors asked to stay in Akaroa and not attempt to drive toward Little River.

Communication networks in some areas are limited, and in an emergency residents are urged to call 111.

Response teams have been deployed across affected communities to check on residents.

In Little River, flooding remains present in and around the township, with limited communications also affecting the area.

A boil water notice has now been extended to Little River, joining Wainui, after flooding caused issues with local water supply infrastructure.

The council said extensive flooding and overflowing septic tanks in Little River had reduced the capacity and availability of the water treatment system.

While nothing is reported to be broken and no contamination has been confirmed, authorities cannot guarantee water quality and the notice has been issued as a precaution.

Residents in Wainui and Little River are being advised to boil all water used for drinking, food preparation, brushing teeth, washing utensils and making ice.

Water should be brought to a rolling boil for 1 minute.

Water does not need to be boiled for showering, bathing or laundry, but people are advised not to swallow water while showering.

Lake Forsyth has now been opened to the sea, with a 50 tonne digger creating a channel. Council staff are monitoring the opening, as high swells or tides could push gravel back into the channel and narrow or close it.

Equipment remains on site to maintain the opening if required.

Residents across Banks Peninsula are also being urged to conserve water while heavy rain continues to affect treatment and production systems. Drinking, cooking and hygiene use can continue, but non essential washing and laundry should be delayed where possible.

The council has issued safety advice for those in flood affected areas, warning residents to stay out of floodwater and not to walk, swim or drive through it. Floodwater should be treated as contaminated, and anyone in immediate danger is urged to call 111.