England did most of the attacking but led only 10-7 midway through the second half after a Ben Earl try, but poured on the power with three more tries by the irrepressible Henry Pollock, Alex Mitchell and Luke Cowan-Dickie.
"What pleased me was the resilience and composure of the players out there," Borthwick told reporters.
"I think probably everyone would say there were a number of line breaks that we didn't quite convert, so we didn't get the scoreboard pressure on, which meant it was a period where the game was tight in very difficult conditions.
"In attack we were probably a little bit over-eager and the combinations had just not quite trained together enough or played together enough, which is why some of those line breaks didn't quite finish the way we wanted them to.
"But I think the players were really clear on what they were attempting to do in Q4."
Five Lions replacements
Key to England putting the game to bed was the introduction of five British and Irish Lions forwards early in the second half, after which Australia barely got out of their half.
"I thought the bench certainly had a positive impact and that's a sign that the squad's building in depth," Borthwick said.
"It's been an important summer for English rugby with the Lions being successful down in Australia and a number away in Argentina and America, it's enabled the squad to grow.
"A big positive there was the last quarter, you feel that this team is going to be strong - we were again today - and there are a number of games we've won in that final quarter."
The man who broke Australian resistance was 20-year-old crowd favourite Pollock, with his new bleached hair, who superbly scooped up a loose ball to charge over the line for the second try to make it 15-7.
"I think everybody's excited when Henry Pollock gets on the pitch, what he brings," Borthwick said of the flanker who scored two tries off the bench on his debut against Wales in this year's Six Nations and made it three in just over half an hour of international rugby.
"I think he's probably the most excited person, the way he is a ball of energy and I love having that character in the squad."
England next face Fiji, who they lost to for the first time two years ago, before taking on New Zealand and Argentina.
"What's most important is that we have a good training week and we step forward again over these next seven days," Borthwick said.
"We probably didn't do our jobs well enough (at the breakdown) today. We know Fiji challenge the breakdown and they have a big counter-ruck threat, so we've got some work to do this week."