Bowel Cancer awareness advocates say GPs are being left in the dark, because screening programme results are too simplistic.
Otago University researchers are calling for an overhaul of the programme, which only tells patients if they have tested positive or negative for blood in their stool samples, based on a cut-off point.
Bowel Cancer New Zealand spokesperson Mary Bradley said a negative result could provide a false reassurance that all was well - even if there was a high level of blood.
"GPs should as a matter of course be sent the test results, because then with their patient they can see if there's a family history, they can see if that patient is presenting with further symptoms and it just gives a clearer picture, it gives them more information," Ms Bradley said.
Bowel cancer was the second biggest cancer killer in New Zealand, so it was important doctors got the best data possible, she said.
Otago University preventive and social medicine researcher Brian Cox said it could be illegal to withhold full results of bowel cancer screening from patients.
Under the Code of Rights, people should be given that information and more detail than just a pass or fail, Dr Cox said.
"The results that have been given thus far appear to have not met the legal requirements of the code of rights," he said.
"People that have had tests done should have the results sent to them now - and that's thousands of people.
"The level of blood in the stool that would be reported as negative by the bowel screening programme in many other countries - Australia and Denmark, for example - they would be referred for a colonoscopy."
Dr Cox said the way tests were reported needed to be immediately overhauled so it complied with the law.
New Zealand has the highest level of bowel cancer in the world, he said.
The Ministry of Health has defended its decision not to give patients their full bowel cancer test results, saying it could just confuse them.
National Bowel Screening clinical director Susan Parry said the programme was in step with international guidelines.
Patients could request their numeric results, but test results had to be reported in a way that was easily understood, she said.
Dr Parry said there may be other medical reasons why there was blood in faeces, and patients were encouraged to go to their GP if they saw blood or if their bowel habits changed.