Alarm bells were first raised last year about billing practices at the Mid-City Skin Cancer Centre in George Street, in Sydney's CBD, where David Lindsay practised between 1998 and December 2007.
Health Care Complaints Commissioner Keiran Pehm says Mr David Lindsay was deregistered two months ago, after authorities received more than 53 complaints about him.
"We had a lot of complaints about rudeness. He threatened to sue people who complained about him," he said.
"People found him intimidating, there were issues of consent about treatment of patients."
The complaints prompted a review into treatment his patients had received. An expert panel looked at the test results of more than 9,000 people.
Mr Pehm says the probe found the former GP had not removed enough of the cancer or surrounding tissue in some cases.
"One patient had a lesion removed from his cheek and subsequently went to another doctor and had it diagnosed as cancerous," he said.
The Health Department's acting chief health officer, Kerry Chant, says inadequate record keeping meant some patients who needed further cancer surgery could have missed out.
"The view is that he adopted a watch-and-see approach in cases perhaps that, in the view of experts, may have required more urgent re-excision," she said.
She also revealed letters were sent to 6,770 patients to advise that some of them may need to follow-up with their GP or specialist, amid concerns about their original treatment.
Of those patients, 1,310 were in a high risk category requiring timely medical attention.
Dr Chant said many of the patients would have already had ongoing care.
"We don't want to overplay the concerns," she told reporters in Sydney.
"We're taking a very precautionary approach. We're providing the information so patients can act on that."
Lindsay himself denies any wrongdoing. He has lodged an appeal against his deregistration that will be heard early next year.
"All of these patients have been managed 100 per cent sincerely, properly and with the intention of curing them and completing treatment each time," he said.
"Each patient has been appropriately followed up until the end of 2007."
Lindsay now answers the phones at the clinic, which he still owns.
"If I get my name cleared, I will come back for two years, just to say, 'Up yours, doctors, I'm back,'" he said.
The clinic is now up for sale.