The controversial Member's Bill, which would create a euthanasia system for competent terminally ill people who opted to use it, passed its second reading in Parliament this week 70-50.
Mr Seymour, in Dunedin yesterday, said as well as changes he had agreed to make, Bill opponent National MP Maggie Barry has suggested she would move more than 100 amendments, a tactic which could mire the Bill in Parliament for months.
Mr Seymour estimated if opponents were truly determined they could make the debate last four days; members' Bills are every second Wednesday and seven remain this year.
"They could maybe even stretch it out for the rest of the year, but not too much further ... but I think the Speaker is going to be quite severe with anyone who seems to be actively wasting time.
"They keep saying they are going to filibuster, but I think that will be harder than they think."
It has been a long road to this point in the legislative process; Mr Seymour had to wait two years for the Bill to be drawn in the Member's ballot, wait for it to reach the House, pass its first reading, then watch it go through a 16-month select committee process.
"None of that was easy, but the next bit will be challenging," Mr Seymour said.
"We are going to need to get 61 votes for critical amendments and 61 votes against vexatious amendments opponents will put up, which is both a massive political and logistical exercise, but the fact we have a 10-vote margin for the Bill right now shows us that it is possible."
Most MPs have been freed from the party whip to vote their conscience on this Bill, but both the Green Party and New Zealand First are supporting it on party lines.
New Zealand First's backing is dependent on the issue being decided by referendum, and Mr Seymour said he would need to negotiate with MPs who backed the Bill but not a referendum.
"We are going to have to work through with people like that, which is more important," Mr Seymour said.
He has a suite of pre-arranged amendments to his Bill, the most significant being making its provisions only applicable for the terminally ill, with the intention that the Bill remains coherent and does not contain so many safeguards that people will not be able to avail themselves of the law change.
"The Bill is already very similar to international best practice ... but there is a danger that you go so far overboard in trying to introduce safeguards that you actually create a nominal legal possibility that is impossible for people to access and that is something we are eager to avoid, as much as having no Bill at all."
Mr Seymour said he was pleased with Speaker Trevor Mallard's careful handling of the debate, and the thorough contributions of all MPs who spoke.
"We were actually seeing politics as school kids are taught it works, that people think hard about the issue and then they go along and cast their vote."
