Troy Hunt reported that a flaw with the electric vehicle's companion app also meant data about drivers' recent journeys could be spied on.
Mr Hunt said he gave the firm a month to fix the issue before he decided to make it public.
Nissan said there was no safety threat.
The problem remains unresolved but Mr Hunt said car owners could protect themselves by disabling their Nissan car wing account. Those who have never signed up are not at risk.
Mr Hunt acknowledged that the issue was not life-threatening, but said hackers could still exploit the app's vulnerability to cause mischief by running down people's batteries.
"The right thing to do at the moment would be for Nissan to turn it off altogether," Mr Hunt told the BBC.
"They are going to have to let customers know. And to be honest, a fix would not be hard to do.
"It's not that they have done authorisation [on the app] badly, they just haven't done it at all, which is bizarre."
A spokeswoman for Nissan said it was tackling the problem.
"Nissan is aware of a data issue relating to the NissanConnect EV app that impacts the climate control and state of charge functions," she said.
"It has no effect whatsoever on the vehicle's operation or safety.
"Our global technology and product teams are currently working on a permanent and robust solution.
"We are committed to resolving the issue as a matter of priority, ensuring that we deliver the best possible experience for our customers through the app now and in the future."
Mr Hunt said the root of the problem was that the firm's NissanConnect app needed only a car's vehicle identification number (Vin) to take control.
The code is usually stencilled into a car's windscreen, making it relatively easy to copy.
The initial characters of a Vin refer to the brand, make of car, and country of manufacture/location of the firm's headquarters.
So, Mr Hunt said, it would only be the final numbers that varied between different Nissan Leafs based in the same region.
"Normally it's only the last five digits that differ," he explained.
"There's nothing to stop someone from scripting a process that goes through every 100,000 possible cars and tries and turn the air conditioning on in every one.
"They would then get a response that would confirm which vehicles exist."
Attackers would not even need to use the app, he added, since the commands could be sent via a web browser.
To confirm the problem, Australia-based Mr Hunt used the Vin number of a Nissan Leaf-owning acquaintance based in the UK.
"I was sat in the vehicle with everything powered off and didn't have my key on me," recalled Scott Helme, who is also a cybersecurity adviser.
"So, the vehicle was as it would be if it was completely unattended.
"As I was talking to Troy on Skype, he pasted the web address into his browser and then maybe 10 seconds later I heard an internal beep in the car.
"The heated seat then turned on, the heated steering wheel turned on. And I could hear the fans spin up and the air-conditioning unit turn on."
Further tests indicated that the hack did not work if the vehicle was in motion.
But it was possible to see the owner's registered username, which might help reveal their identity.
Furthermore, times and distances of recent journeys were disclosed, but not location data.
As soon as Mr Helme unregistered his app, Mr Hunt could no longer contact his car.
"It's not as bad as it could be," Mr Helme told the BBC.
"But if I was to monitor your movements over the course of the week and learn when you go to and from work, shortly after you got to your office I could run the heating for the remainder of the day.
"That would potentially leave you with very little power - certainly not enough to get back home."
Further analysis indicated that the app does not talk directly to the cars, but instead sent its commands via Nissan's computer servers.
As a result, Mr Hunt said, it would be easy for Nissan to suspend the service.
The researcher also discovered that some Canadian owners of the Leaf had discovered and shared knowledge of the flaw on an online forum and had posted a web address that could be used to spoof the app.
"I decided we were past the point of not letting the cat out of the bag," he said, justifying his decision to blog about the discovery before Nissan had issued a fix.
"Unfortunately what we are seeing is just another case of security being important after a problem is discovered," he added.