Water bills in Auckland and Wellington are about to climb, as water companies try to catch up on decades of under-investment.

In Auckland, Watercare revealed on Thursday an average household's water bill would go up about $8 per month, as part of a $15.2 billion investment in the city's water infrastructure.

Meanwhile in Wellington, people would pay 13.3 percent more as new water agency Tiaki Wai invested in ageing infrastructure.

Auckland has been plagued by problems with its ageing water infrastructure, including in 2023 when a huge sinkhole in Parnell meant millions of litres of sewage a day were pumped into the Waitemata harbour.

Deputy Mayor Desley Simpson said Watercare's $15b investment would prevent those kind of problems in the future.

"It means that we're going to get investment that we've been crying out for ages, below the ground to improve water quality... and actually finally gets us an Auckland that we can swim at our beaches with no black flags."

Separating Watercare from Auckland Council, with its own balance sheet, had meant it could take on more debt to fund the infrastructure, Simpson said.

Watercare has also put up water prices, by about $8 per month for the average Auckland household, to help pay for it.

"And that's painful. It's a tough economic time. But I suppose one of the advantages we have in Auckland is we can, as households, monitor how much we spend. You know, if you have shorter showers," Simpson said.

Aucklanders RNZ spoke to agreed the increase would be painful.

"Well it affects everyone doesn't it? To a degree. I think I could afford $8, but a lot of people can't," said one.

"For lower-income people, it probably would be a challenge for them, so I feel sorry for them in that regard," said another.

But many felt the extra cost was worth it, if it meant fewer issues with the water network in the future.

"I sometimes go to Mission Bay and you can't swim there because there's some warning sign saying that it's contaminated. So I definitely look forward to that."

"It's heartbreaking really, we're supposed to be this clean, green country and summer after summer not being able to go to the beach."

In Wellington, council organisation Tiaki Wai has taken over management of water services, and sent out its first new bills on Wednesday.

Water bills in the city would increase by 13.3 percent on average this year.

Wellingtonian Charlotte Murphy felt there hadn't been enough communication that the new bills were coming.

"I haven't seen a lot of information as a ratepayer about it, and I know that there are a lot of questions around it from people in, for example, my apartment complex."

Back in Auckland, Watercare chief executive Jamie Sinclair told Checkpoint the infrastructure work it was doing was urgently needed.

"We've got significant underinvestment in the assets that we own. The 20,000 kilometres of pipe or so that we maintain at the moment. We also need to build resilence into our network as we're dealing with more extreme weather events and also increasing demand."

Sinclair said now that funding was locked in, procurement would start on some major new projects by the end of the year.