The 20-minute session was run in drying conditions, with the slippery circuit catching out a host of drivers.
An 11-car field was quickly reduced to 10, with Tom Alexander’s Colorado stopping just three corners after leaving the paddock.
“On the outlap, the car just died – there was no power to the dash or any of the electrical systems,” Alexander said.
“MoTec are examining the system to see what caused the issue, so we can make sure it doesn’t happen again in qualifying or the races.”
The field was brought to a standstill as the red flags were rolled out, before the session resumed with 13 minutes remaining.
As times improved, the order changed rapidly until EFS 4x4 Mazda driver Harris put in a 1:46.5745s with just under two minutes left.
Aaron Cameron made it a Mazda one-two, finishing 1.0543 seconds off Harris in the other BT-50, followed by Craig Dontas in his factory-backed Mitsubishi Triton.
“It’s obviously good to be a second clear of the field in practice, but I don’t think today’s times are an indication of what will happen once it dries out – I think it will be a lot closer,” Harris said.
“We didn’t do the two-day test here last month, so everyone else has more dry track knowledge than we do.
“At the end of the session, I pushed a bit too hard and hit the ripple strip coming out of Turn 12, which caused me to have a big slide.”
Ben Walsh led the Toyotas in fourth, ahead of series leader Craig Woods while Matthew MacKelden was seventh in the third Hilux.
Dakar Rally winner Toby Price improved from the opening practice session, placing his Mitsubishi sixth for Team Triton.
AC Delco's Tomas Gasperak and Isuzu driver Stephen Wilson occupied eighth and ninth, followed by Christopher Formosa in his Ford Ranger.
“The driveshaft and diff assembly broke, which we think was a result of a bolt being weakened from the crash we had at Adelaide,” Formosa said.
“We’re examining the car at the moment – as long as the tailshaft is not bent, we should be all good for tomorrow.”
ARMOR ALL Qualifying will take place at 12:05pm local time Saturday, followed by Race 1 later that afternoon.