The second round of talks to pull together a new government will kick off this morning and could stretch into the night.
Both National and Labour will again front teams to meet with New Zealand First to make their respective cases for why they should be at the helm.
National's negotiations are due to resume in Parliament at 9.30am; Labour's at midday.
Both meetings on Sunday lasted just over two hours each.
NZ First leader Winston Peters described the talks as "very constructive" and expected them to continue on Monday "all day ... and into the night".
Negotiations were proceeding "policy by policy" and ministerial positions had not yet been discussed, Mr Peters said.
"Let's sort out the things that really matter. Policy's everything. Otherwise it's just about personalities, isn't it?"
He said there was nothing to suggest NZ First would not meet its self-imposed deadline for a decision by Thursday.
Mr Peters said he was still weighing up all the possible arrangements when it came to forming a new government.
"I'm not trying to be difficult with anyone in the media. But if you don't keep your eyes wide open to all the possibilities, you may miss a great chance for your country."
That meant any deal - including a full coalition, confidence and supply deal, or sitting on the cross benches - was still on the table.
"Just like I said day one," Mr Peters said.
Security at Parliament was heightened on Sunday.
Media were barred from filming the meeting room's entrance and reporters had their security access to the nearby hallway revoked.
At one point, a security guard tried to physically move journalists as Mr Peters and his negotiating team left the building for lunch.
Press Gallery chairperson Katie Bradford expressed concern and said she believed parliamentary security had "over-reached".
The Speaker's Office said it did not sanction the changes made to security around the precinct.
Mr Peters had said he would not begin official negotiations until after the final election result was revealed.
The final tally - released on Saturday - saw National drop two seats to 56, Labour up one to 46 and the Greens up one to eight.
Both National and the Labour-Green bloc require New Zealand First's nine seats to govern.