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It is hoped a wastewater mains renewal programme will reduce sewage overflows into rivers and roads in Christchurch.
The wastewater system struggled during heavy rain this week, and it was the more modern sections that buckled under pressure.
Residents in areas with a vacuum system were told not to flush their toilets during the height of the deluge, as it grappled with stormwater infiltration.
Christchurch City Council head of three waters Helen Beaumont said the vacuum systems have not been working optimally.
"The new parts are more robust in terms of the pipe and the pumpwork. However, the way that we connected into some of the properties and the way the vacuum systems particularly can draw in stormwater, has been a challenge to us," she said.
The system still managed to process high flows but not without fault, Beaumont said.
"We've still managed to maintain very high flows through our wastewater treatment plant, it's just that those high flows are surcharging back through the wastewater system, so we're seeing a number of overflows into creeks and rivers and some overflows into roadways as well."
The vacuum systems were installed in some suburbs after the earthquakes, but the majority of the city's system was gravity based.
A specific group will be set up to look at vacuum system improvements, which appear to be more susceptible to stormwater intake than the main gravity system, Helen Beaumont said.
She said the stormwater system held up well, even near capacity.