Pauline Hanson's One Nation party has officially won its second Senate seat, claiming one of two final Senate spots in Western Australia. The other seat was won by incumbent Greens senator Rachel Siewert.

The result leaves farmer Rod Culleton poised to join Ms Hanson in the upper house – although a conviction in NSW and a separate charge in his home state threaten to render him ineligible to serve.

It comes as the Australian Electoral Commission prepares to complete Senate preference counting in the four remaining states, with the final batch of results expected to be announced on Wednesday.

A crossbench of at least 10 senators now appears likely, including three One Nation representatives, Derryn Hinch in Victoria, Jacqui Lambie in Tasmania and possibly two Liberal Democrats – one each from NSW and Queensland.