The revelation comes the day after a TAIC report into a fatal plane crash at Fox Glacier in 2010 found two commercial skydivers on board had cannabis in their systems.
That prompted a safety recommendation to the Transport Ministry, urging the introduction of a drug and alcohol detection and deterrence regime for people employed in safety-critical transport roles.
At present, there is no legal requirement for random drug testing of the likes of commercial balloon pilots or skydivers.
Mr McClelland said it would be up to the ministry and the Civil Aviation Authority (CAA) to determine how a drug testing regime should be implemented.
He would not reveal the level of THC, a cannabis constituent, found in Mr Hopping's system.
Levels varied widely between individual cannabis users and factors included frequency of use, potency and smoking methods.
"There are simply too many variables to state a figure without giving the background information, and further research needs to be done to confirm both the figure and put it in context,'' he said.
"We need to have the results analysed again independently. We need to put it in context in terms of how long the autopsy took, the results took, and also the background of the pilot himself.''
The results of alcohol testing on the pilot were inconclusive.
Mr McClelland would not say how the families of the victims reacted to news of the positive drug test, saying the TAIC had met with them in private and it was inappropriate to comment.
CAA chief executive and director Graeme Harris said he was "very disappointed'' by evidence of drug use in recent TAIC reports.
"The CAA is planning an education and safety promotion campaign on this issue, consistent with the advice provided to the commission by the Ministry of Transport in its report into the Fox Glacier accident released yesterday.''
Responding to that advice, Transport Ministry aviation and maritime general manager Bruce Johnson said changing individuals' illegal drug use was challenging.
The ministry would encourage the CAA to step up its education efforts, but in the past it had been unconvinced that the benefits of compulsory random drug and alcohol testing outweighed the cost.
"Specific rule changes may not always be the best interventions to achieve desired safety outcomes. Non-regulatory interventions can often be more appropriate.''
The ministry would monitor international testing regimes, including recent aviation rule changes in Australia.
The families of the victims, who have been briefed on the interim report, have asked for privacy from the media.
The Masterton locals killed in the crash were Mr Hopping, Valerie Bennett, 70, and husband and wife Desmond and Ann Dean, aged 70 and 65.
The victims from the Wellington region were Ms Bennett's cousin Denise Dellabarca, 58; husband and wife Howard and Diana Cox, aged 71 and 63; Lower Hutt couple Stephen Hopkirk, 50, and Belinda Harter, 49; and young Wellington couple Chrisjan Jordann, 21, and Alexis Still, 19.
Drug use 'completely unacceptable' - PM
Prime Minister John Key today said it was completely unacceptable that cannabis had been found in the systems of the balloon crash pilot and the two skydivers.
Mr Key, who is also Tourism Minister, told media at a tourism event in Queenstown that the Fox Glacier crash likely would not have happened under new civil aviation rules that came into effect last year, Radio New Zealand reported.
Labour tourism spokesman Rino Tirikatene said there were concerns new rules were being implemented too slowly.
"It is time for John Key as Tourism Minister to step in and make sure the agencies involved have the resources they need to move more quickly to address safety concerns.''
Mr Key also refuted "facts'' used by the father of a Fox Glacier plane crash victim to claim on the internet that New Zealand is unsafe for tourists.
Chris Coker's 24-year-old son Bradley was one of nine people killed in the Fox Glacier crash. Mr Coker has said in a YouTube video and social media campaign that his son's death was "completely avoidable'' and showed a lack of proper regulation and control.
Mr Key said today that he wanted to pass sincere condolences to Mr Coker for the loss of his son, "an enormous tragedy, so I feel for him as a parent and as a politician.
"He needs to understand, as others do, that we take it very seriously in terms of improving safety standards. My fundamental view is that the industry is safe, for the most part it was safe, the vast bulk of operators have operated for a long period of time and the real changes we've made are about eliminating one or two rogues that tarnished the whole industry.
"What is not true is some of the claims made in terms of the number of deaths and the state of the industry. They are just factually incorrect, but I can understand his pain and we're committed to doing a better job.''
The Minister of Transport Gerry Brownlee said there was sufficient evidence to support a call for compulsory drug-testing for those involved in transporting people.
"I've asked the accident investigation committee to give us a bit more information about how often this is happening in these accident situations,'' Mr Brownlee said.
"Any time you've got people in charge of a vessel or vehicle with passengers in it we have to be very concerned about their state of fitness and this is a big concern.
"It's a shame that you will always get a small percentage of operators who will transgress like this. It's incumbent on us to try and tidy things up,'' he said.
