The prime minister on Tuesday described the move as the biggest change in national security for four decades.
Under the plan, which would come into force next year, a home affairs portfolio headed by Immigration Minister Peter Dutton would cover agencies including the Australian Federal Police, ASIO, Australian Border Force as well as customs and citizenship.
Mr Abbott says the issue was raised when he was prime minister but he was advised against it.
"The advice back then was that we didn't need the kind of massive bureaucratic change which it seems the prime minister has in mind," Mr Abbott told 2GB radio on Wednesday.
"I can only assume the advice has changed.
"No doubt the prime minister will give us more information in due course about the official advice that he's had on this."
Mr Turnbull's plan could face some hurdles, given some elements require parliament's approval.
Labor leader Bill Shorten has described it as a "rushed proposition", demanding more information.
Mr Turnbull insists the merged department - modelled on the UK's Home Office - would make Australians safer.
"Set and forget has no place in national security. Complacency has no place in national security," he said.
But any changes to the AFP, ASIO and other agencies need to go through parliament.
A task force would work on the necessary legislation and other changes needed to implement the reforms and Mr Turnbull said he hoped Labor would support them.
However, Mr Shorten - who spoke with the prime minister on Tuesday night about the proposal - said it was "one of the more rushed propositions we've seen in national security".
He said security agencies had in the past expressed reservations about such a restructure.
"I take my advice from the experts on the front line of fighting terrorism, not the government who looks like they're just trying to keep Peter Dutton happy, give him some more power, in order to protect Malcolm Turnbull's job," Mr Shorten said.
He said he had little confidence in Attorney-General George Brandis, who under the changes would be in charge of more oversight agencies including the ombudsman.
The author of a review of the Australian intelligence community, former senior public servant Michael L'Estrange, said there was still much work to be done.
"I think it's got a way to go in terms of how the flesh is going to be put on the bones, how the governance will work," he said.
The former head of the foreign affairs department recommended in his review the creation of an Office of National Intelligence to better co-ordinate and oversee activities across 10 agencies and advise the prime minister on security issues.
Mr Dutton said the new portfolio would make Australians safer because his sole priority as a cabinet minister would be national security.
He said agency heads were consulted and it was not done for political reasons.
