Chief of the Defence Force David Johnston told a senate estimates committee three used Virginia-class submarines, to be acquired through the AUKUS pact, would be "highly capable".
The $368 billion plan originally had Australia receiving three nuclear submarines from the US - two used and one new Virginia-Class vessels - before building its own in Adelaide.
"These are the best attack submarines in the world, with the first time it has ever been transferred from another nation," Admiral Johnston said.
"These are extraordinarily capable platforms that will come to us with a long life remaining in that submarine fleet, and it would be inaccurate to portray it any other way."
Independent Senator Jacqui Lambie said pact changes had left Australia with "trash".
"You are misleading the Australian people. We're not getting what we wanted," she said.
"Right now, the Australian population is saying you and the Labor government failed."
Senior Defence officials, including the Chief of Navy and incoming Chief of Defence Force, Vice Admiral Mark Hammond, confirmed it had long been the view of industry that second-hand submarines were the best option.
"It seems to be that this is a much better plan. It is lower risk, more cost effective and more likely to succeed," he said.
His comments reiterated that of Defence secretary Meghan Quinn on Tuesday night.
"Australia's position is that we would have always ... had a preference for three in-service (submarines)," she said.
Foreign Minister Penny Wong said it did not matter whether the submarines were used or new.
"Whether it's two (used) and one (new) or three, it's the capability that matters," she told ABC Radio on Wednesday.
"We want three submarines to deal with, from the United States, to deal with a capability gap before the AUKUS submarines are to be delivered ... that is the plan."
Defence Industry Minister Pat Conroy said the change in the AUKUS deal did not mean a fundamental altering of the security pact.
"It will be cheaper, simpler to manage, and it's been confirmed by the Pentagon overnight," he told ABC Radio.
"We'll get submarines ... about six years into their 33-year life cycle. They'll be cheaper, they'll be really effective at that stage, and we'll be acquiring the most capable nuclear-powered conventionally armed submarines in the world."
Mr Conroy said Australia would save a "considerable" amount by not acquiring a new submarine, but did not disclose the cost.
He denied the used submarines would be more costly to maintain in the long term.
The comments come after Labor backbencher and former minister Ed Husic called for the government to rethink the multibillion-dollar plan.
Mr Husic said on Tuesday the deal also had to be rethought due to America becoming a more unreliable ally.
"You do wonder whether or not we will get the deal, even the reconfigured one that we have got," the western Sydney MP told reporters at Parliament House.
Senator Wong said the backbencher was entitled to his view on AUKUS.
"It is in the best interests of our country for this project to continue to proceed. We believe it is necessary for Australian security and we believe chopping and changing will only set the country back," she told ABC TV.
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