He rejected suggestions the government's main aim was to save Prime Minister Gordon Brown from a humiliating Commons defeat, saying it was "trying to get good law in this very delicate and serious area."
But Downing Street played down talk that the government was heading for victory.
The prime minister's spokesman said it was "certainly not the case that this vote is in the bag and there is more to do over the days ahead".
Ms Smith addressed doubters in a meeting of Labour MPs on Monday evening and in a string of media interviews.
She insisted safeguards would ensure the proposed 42-day limit would be used only in "grave and exceptional" circumstances.
She told BBC Radio 4's Today programme: "Any home secretary using these powers will want to think very, very carefully about how they are going to explain it to Parliament, whether or not they are going to get parliamentary approval."
MPs will also be given "independent legal advice" about the home secretary's decision "to help to inform that debate".
Ms Smith said some MPs, understood to be members of the Intelligence and Security Committee, will get confidential briefings on a "privy council basis" about individual cases before the debate takes place.
'Attempt to deceive'
"What I'm concerned about is that we do the right thing for the UK, the right thing for people's rights, and that is that we avoid a terrorist attack as well as the rights that we completely and importantly have to afford to defendants," she said.
"Finding that balance is a difficult job: that's why we've been talking about it for so long."
But the Liberal Democrats' Chris Huhne said his party would continue to oppose the plans "tooth and nail".
He said: "Jacqui Smith's so-called key concession is a con. All she has to do is to tell Parliament that there is a grave and exceptional terrorist threat, and she will be entitled under the proposed amendment to go ahead without even considering whether the proposals are proportionate to the problem."
And shadow home secretary David Davis said the amendments were "a politically driven attempt to deceive Parliament".
He said: "These amendments are a deception. The home secretary's power to extend the period of detention without charge to 42 days remains as wide as before - and is certainly not confined to a state of emergency."
Speaking after Monday's meeting, Aberdeen North MP Frank Doran, who had planned to vote against the government, said believed the ground was "shifting" among potential Labour rebels.
But Labour MPs John Grogan and David Winnick were still not convinced the case had been made to extend the 28-day limit.
The vote may hinge on smaller parties such as the DUP, which voted against the government on 90 days but has yet to decide on 42 days and the new package of amendments.
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