A Christchurch beachgoer claims a popular swimming spot that's been declared unsafe is perfectly fine for a dip.

Dan Abel is challenging Environment Canterbury's findings that Corsair Bay in Lyttleton Harbour should be avoided.

He said the agency's sampling is too irregular and his own tests paint a different picture.

Abel has been using the same lab as the regional council to check the quality of the water himself.

"We've done ten tests so far...and all have tested superb. The conditions have been very stable so we're confident but we're also a little bit confused, because our number one advisory tells us the opposite to that, so that's something we've got to sort out," said Abel.

Ocean swimmer Leith Cooper said he's been helping to pay for the daily tests.

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"I understand they need a long term risk assessment, we understand that, but we swim daily and so we need that kind of reporting on a daily basis so that we can make informed decisions," said Cooper.

Abel concedes that "on occasion there will be pollution sources" after rainfall, and from humans and dogs.

"Those pollution are spikes, they're gone within 48 hours and what we need is more regular testing to identify when they occur," said Abel.

In October, Environment Canterbury ruled the bay was "unsuitable for swimming" this summer, using data collected over five years showing high levels of bacteria including E. coli.

Science Team Leader for Water Quality and Ecology, Shirley Hayward said samples are generally collected weekly from late November to early March, and the no swim rule was "ultimately a question of human health".

"The long-term grades are based on the routine weekly water samples taken across the previous 5 seasons. This means that a long-term grade can be determined from around 75 samples.

"These long-term grades are reassessed annually prior to the start of the swimming season.

"We include high-rainfall data for long-term grades for bays, harbours or lakes because, unlike rivers, swimmers aren't usually deterred from getting in the water at such sites after rainfall. For these sites, unlike rivers, there are often no visible signs of recent rainfall.

"It's also important to reiterate that samples at Corsair Bay have indicated an elevated risk even without recent rainfall events," said Hayward.

Dan Abel told 1News he will continue to test Corsair Bay and will publish the results on his website.