Chief Commissioner Ken Lay has confirmed that an arrest was made last night and charges were expected to be laid soon.
"We have made some significant breakthroughs in that investigation overnight," Mr Lay said on radio station ABC 774.
Operation Resound was set up to probe an escalating battle between the Hells Angels and Bandidos after Mitchell was shot in an ambush during March outside a bikie clubhouse in Melton. It was the second attempt on his life, after he was shot in Brunswick in 2011.
Fairfax Media revealed that police feared the gangs were on the brink of "all-out war" after the shooting of Mitchell, the Bandidos serjeant-at-arms.
Mr Lay said Operation Resound police had issued about 35 warrants and arrested 25 people on a range of offences including drug and firearm charges.
"This has been working very hard to dampen down tensions between the Bandidos and Hells Angels," he said.
"What we are starting to understand is the insidious reach of outlaw motorcycle gang."