A random audit conducted by Whangarei District Council found 10 out of 11 pools inspected were non-compliant.
The discovery has promoted a full review of all the swimming pools in the council database, amid concerns there are other pools that recieved approval in the past even though they didn't comply.
Whangarei has about 1500 private swimming pools.
Chief executive Rob Forlong said that early in March, council was contacted by an owner seeking code compliance for their property before it went on the market.
"During our inspection we discovered that the property's pool fence, approved several years ago, was not compliant and that it would be possible for youngsters to get through it and get into the pool area," Forlong said.
Concern that this may be the case on other properties prompted council to randomly audit - at no cost to the owners - a further 11 pools which had been approved in the past. They found that 10 of them were not compliant.
"Do not rely on the fact that you have a certificate for your fence, you must still check," Forlong said.
"Pool owners have a legal responsibility to ensure their pools meet the requirements of the Building Act, regardless of when they were last inspected and approved," he said.
"Fencing rules are entirely about preventing drownings, so we want people to take nothing for granted about their pool fence."
Forlong recommended pool-owners looked for places where vegetation may have grown up and also or changes to buildings on the property and the properties next door.
Ground level changes, excavation, garden beds, new plantings or sheds or general wear and tear may have made it easier for children to get through or over the fence, he said.
"It is autumn, and many people will not be using their pools at the moment, but they will still be full of water and we all know water is very enticing to children."
"Anyone who has any concerns about their fence is welcome to contact us for advice, and we will be contacting all pool owners on our database."
More information about pool fencing rules and responsibilities is available on Council's website and Facebook page and people are welcome to call (094304200) for advice.