Kamau's goal was his first in the A-League and it was the last word in a tight tussle between these two traditional rivals in front of a crowd of 18,924.
Kamau's shot took a wicked deflection off Victory defender Scott Galloway, but nobody can take the winning goal away from the 20-year-old, who only minutes earlier came off the bench to replace Bruce Djite.
Adelaide remain third, but now just one point behind Western Sydney Wanderers and with a four-point buffer to Melbourne City.
Victory though had the better of the first half and will be disappointed not to have taken an advantage into the break.
Archie Thompson, making a rare start in the absence of Kosta Barbarouses had a sight of goal but he fired over the bar.
Eugene Galekovic had been busy between the Adelaide sticks but he had the crossbar to thank for saving him shortly before half-time.
Gui Finkler, fresh from his omission from Victory's AFC Champions League squad, had a point to prove and when he lined up a free-kick from some distance he only had one thought on his mind.
Finkler was probably still smarting too from the free-kick which crossed the line, but wasn't given, in last week's derby.
He curled the ball beautifully around the wall and Galekovic for once was beaten, but the ball crashed back off the bar and to safety.
The Adelaide 'keeper had to be at his best to deny Thompson from a tight angle in stoppage time, the incisive pass coming from the hyper-motivated Finkler.
Adelaide found themselves in the second half and they began to pose a few more problems for Victory.
Craig Goodwin found himself in on goal but with no support and from a tight angle, he had no choice but to shoot.
So, shoot he did, and the crossbar this time came to Victory's rescue.
Adelaide found the back of the net eight minutes from time, but Djite's clever lob was ruled out for offside.
He immediately made way for Kamau - and the rest is history.
If there was a cloud over Adelaide's success it was a heavy blow suffered by the excellent Craig Goodwin in the lead-up to the winning goal.
Goodwin was tackled from behind by Rashid Mahazi and copped the sliding knee of Daniel Georgievski to the head as he went to ground in a sickening clash.
Amor confirmed Goodwin was hospitalised after the concussion and would undergo scans.
The Spaniard said there was no secret to his side's success, after a rough start to the campaign.
"It's the work and the work and the work," he said. "And belief.
"We have fantastic players with a winning mentality."
Victory stay fifth with the defeat, and could end the weekend up to nine points off the pace.
Muscat said he witnessed a "very ordinary day at the office" that he didn't see coming.
"It was nowhere near good enough," he said.
"From start to finish, maybe apart from the first five minutes, it was a pretty lifeless performance."
Melbourne Victory 0 Adelaide United 1 (Kamau 90')