On a strong day for Triple Eight after teammate Broc Feeney had earlier set the top time in the first session on track, Brown showed why he is being targeted to replace van Gisbergen when he set the fastest time of the day in a rain-hit session at Mount Panorama.
After being red-flagged late when Will Davison went off the track, Brown emerged on top amid a flurry of flying late laps with his 2:07.55 effort around the mountain, while Finney dropped back to third quickest.
Defending Supercars champion Brodie Kostecki in the opening round, Todd Hazelwood, made the most of his call-up to the title-winning Erebus Motorsport team to finish second in Friday's final session.
Brawn has defected from Erebus to join rivals Triple Eight and take triple Supercars champion Van Gisbergen's seat after the Kiwi star moved to NASCAR racing in the US.
The 25-year-old and Feeney will form the youngest pair on the grid in what team boss Jamie Whincup described as the most exciting driver line-up in the field.
Brown, who finished fifth in last year's championship, said it was a strong start for his new team.
“It felt good in my lap,” Brown said after practice.
“It's definitely different (in a new team). It's a very big change this year for Red Bull Ampol (racing) and I noticed that. But it was really cool.
“I think I have the tools I need to succeed this year and I think it's great to work with Broc and it's been a good start to the year.”
Hazelwood was called in to fill Kostecki's seat in the opening round following the champion's falling out with Erebus and could remain there for longer amid the unresolved saga.
He said he was determined to make the most of the opportunity he was given.
“Just taking every lap and every opportunity in front of me and taking it with both hands,” Hazlewood said.
“I'm really lucky to be tied to a more than capable car, the team is more than capable and really lucky to have some great engineers and mechanics who have given me a great chassis.
“I felt really comfortable in the wet and dry and I didn't take a lot of risks or anything like that. We all know how hard it is touring here at the best of times, so when you can do it confidently and comfortably in both situations, it's a shot in the arm full of confidence.” .
“I'm just trying to do my best and do my best job. Now, wherever this gets me, I don't know.”
Grove Racing's Matt Payne finished fourth in a Ford Mustang, while Matt Stone Racing's Nick Percat rounded out the top five.
It was an exciting start to the final practice session after rain fell on Mount Panorama shortly before the drivers arrived on the track.
The session was red flagged after just six minutes when Anton De Pasquale and David Reynolds became victims of a slippery track at the bottom of the Chase.
De Pasquale was the first to slide on the gravel before Reynolds followed him on the same part of the track, sending his Camaro spinning into the wall.
The session was halted for approximately 15 minutes until the cars were recovered.
Reynolds, who was driving for the new Team 18, recovered to finish 12th in the session, just ahead of Di Pasquale.
Walkinshaw Andretti United's Ryan Wood was the best rookie, finishing 11th fastest.
While Hazelwood pushed his car into the top two, Brown's replacement in the Erebus, Jack Le Brocq, returned to 22nd after earlier engine problems in the opening session.
If the off-season wasn't dramatic enough for the Erebus Motorsport champions with the fallout from Kostecki, the team ran into trouble on the opening lap of the first practice session.
Le Brocq barely managed to get the car on track when he had to park his Camaro quickly at the cutoff after a sudden drop in oil pressure.
Le Brocq had to return to the pits and spent the rest of the session in the garage.
The team was then forced to replace the engine in Le Brocq's car between the first and second practice sessions.
As he set the pace in the opening session, Feeney said he immediately felt strong on the track after a few laps of the track last weekend in the Bathurst 12 Hours.
“It was a good day, and I felt good out of the gate.”
“It was a strong pole position and that (second) session and then the conditions were very strange… I probably didn't expect to be here towards the end of the session, but I'm excited, the car started really strong and I'm definitely looking forward to tomorrow.”
There are qualifying, a top-10 shootout, and a 250km race for the drivers on Saturday.
