A TNS Conversa survey of health insurance customers found four out of five had consulted the internet about medical matters - up 8% from two years ago - and many of them said they would use their findings to challenge their doctor's advice.
The survey of 566 Southern Cross members found more than half relied on the internet as a regular source of health information, making the web second only to GPs as the most relied-upon source of medical advice.
Young people, aged between 20 and 29, were almost as likely to regularly consult the internet as they were to consult their doctor, about health problems.
The survey, carried out in August and issued this week by Southern Cross, supports other studies suggesting people are increasingly turning to the internet for medical advice.
A 2009 Netguide-Telecom broadband survey found a third of respondents used the internet rather than visiting their GP.
Medical Association chairman Peter Foley said it was fine to consult the internet before a doctor's visit, but said many people were worrying needlessly after giving themselves a wrong diagnosis.
"Many people will be falsely worried and others will be falsely reassured by what they do or don't read," he said.
"There is so much [information online], trying to sort out the truth from the bog is an issue."
For those aged 54 or younger, the only other source of advice to come close to the internet in the latest study, apart from GPs, was family and friends.
Those aged 55 or over were slightly more likely to turn to a doctor than the internet.
Younger people were more likely than older folk to consult Google, 90% of 20- to 29-year-olds saying they had used the internet for medical matters, compared with 75% of people aged 55 or older.
More women (86%) than men (75%) had turned to the internet.
Dr Foley said it was dangerous to self-diagnose, even using a reputable health website.
"Health is a very complex subject and there are dangers in interpreting signs and symptoms. If people want to challenge [their doctor's diagnosis], they should do that in the consulting room so that it can be explained."
It was common for people to visit the GP armed with internet data.
"We understand that when we give people labels and diagnoses, they will look it up on the internet. We often direct [people] to appropriate sites."
Southern Cross chief executive Ian McPherson, a former doctor, said online research was positive if it helped people prepare questions for their GP, but he would be worried if it was used as a substitute.
- Eloise Gibson
