Sebastian said the move was a "direct threat" from Titus Emanuel Day who he was suing for money allegedly owed, and had just alerted him to unpaid royalties in 2020.
"I knew Day would stop at nothing."
The musician filed his lawsuit against his long-time manager in July 2018, but approached a detective two years later when some "things came to light".
This was his signature allegedly forged on documents with Premier Music for "neighbouring rights" he only discovered upon trying to sign with the company himself, he earlier told the NSW District Court.
"A lot of this I couldn't get answers ... didn't have solid evidence until I received the Premier Music emails," Sebastian said.
"Things in there I deemed to be criminal and I realised it was the right thing to do."
Defence barrister Dominic Toomey SC suggested Sebastian approached police two days after being served with the AVO to avert the application.
"Absolutely not, I didn't need to," he responded.
The singer said Day in that AVO was also trying to weaponise an incident in which Sebastian had told him that he once chased and "headbutted" an intruder trying to break into his family home.
The Crown alleges that Day - who managed Sebastian via 6 Degrees between 2009 and 2017 - embezzled Sebastian out of almost $900,000 between 2013 and 2020.
Day, 49, has pleaded not guilty to 50 charges including fraudulently embezzling money allegedly owed to Sebastian through royalties and performance fees.
He also denies 50 alternative counts of larceny, or stealing.
On Tuesday Sebastian was taken to his final meeting with Day alongside both their wives in November 2017.
"(There were) a lot of emotions," Sebastian said.
He said that year had been contentious for both as each acknowledged their relationship "was on tenterhooks".
Months earlier Sebastian had embarked on a European tour in support of former boyband member Shane Filan, but pulled out before completion.
"I had reached a not-so-great place.
"The tour itself was an element of it ... wasn't really the right tour but I was also struggling with mental health issues at the time."
Sebastian had been advised by Day that he would be paying costs for the tour, but it was a good opportunity on the back of releasing an album.
In an email Sebastian acknowledges it is a "big investment" for him, but it won't hurt to get in front of "that many people".
He was later shocked seeing a document that showed a purported additional fee was paid on his behalf for the tour.
"Would you have agreed to pay Mr Filan or his promoter for the opportunity to play with him … Have you ever paid in the past as a support act?" prosecutor David Morters SC asked.
"No, I had never heard of that."
The court was previously told that the musician was owed $494,360 for accompanying Taylor Swift four times on her Red Tour in Australia in 2013.
Upon examination of his financial records, Sebastian said half that fee owed was missing.
"There was no payment (of that amount). None."
And back in 2014 when his accountant tried to reconcile the full payment with help from this then-manager, the situation was equally fraught, he said.
"It was like getting blood out of a stone," Sebastian said.
The money was allegedly used by Day to purchase shares in My Medical Records, something Sebastian recalled him heavily promoting at the time.
The trial before Judge Timothy Gartelmann continues.
