A fourth Australian senator has been referred to the High Court to rule on whether he was legally elected due to a constitutional prohibition on dual citizens becoming lawmakers.

Malcolm Roberts said he was confident he had been eligible for election, but had asked his anti-immigration, anti-Muslim party One Nation to refer him to the court on Wednesday "in the interests of honesty and transparency."

Roberts had been under intense media scrutiny before he revealed last month that he only received written confirmation that he was not a British citizen five months after he was elected in July last year. But he has declined to say whether he had ever been a dual citizen.

Roberts declined to say when he lodged a document renouncing British citizenship.