One Nation's Warwick Stacey won the sixth slot in NSW after the Australian Electoral Commission declared the results for the state on Friday.
Labor was expected to pick up the final seat in NSW, but a strong flow of preferences meant the One Nation candidate was able to get over the line.
It means Pauline Hanson's party will have four senators in the next parliament, equalling its high watermark from 2016.
Mr Stacey will join Senator Hanson alongside re-elected Queensland senator Malcolm Roberts and the newly elected Tyron Whitten from Western Australia.
The other five senators elected from NSW included Labor's Tony Sheldon and Tim Ayres, Liberals Andrew Bragg and Jessica Collins as well as Mehreen Faruqi from the Greens.
Labor will have 28 senators in the 76-seat upper house, with the coalition having 27, the Greens with 11, One Nation with four and the remaining six being independents or from minor parties.
A total of 39 votes is needed to pass laws in the Senate, meaning Labor can ensure passage of bills with the support of just the Greens or the coalition.
It comes as the Australian Electoral Commission launched an investigation after almost half of all votes from a polling place were ruled as informal.
Of the 111 people who voted at a polling booth in Missabotti near Coffs Harbour in the seat of Cowper, 50 ballots for the lower house were filled out incorrectly.
Residents from the area told the ABC they were informed by election staff to number both ballot papers from one to six, despite there being 11 candidates.
Ballots for the House of Representatives must have all boxes numbered in order of preference to be deemed valid.
A minimum of six boxes must be filled out on the Senate ballot paper for the party of the voter's choice.
The high informal votes would not have affected the outcome in the seat, with Nationals MP Pat Conaghan winning by more than 5000 votes.
"If the reports from Cowper are accurate and our staff were providing incorrect instructions, this is disappointing," an electoral commission spokesman said.
"The AEC takes ballot paper formality very seriously - our intention is always to maximise the number of voters who are able to participate in a federal election by casting a formal vote."
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