Labor’s interpretation of the “reasonable steps” test has been labelled “radical” and its case to prevent disqualification of Katy Gallagher has been met with scepticism by the justices of the high court.

The solicitor general, Stephen Donaghue, on Wednesday urged the high court to take a black-letter approach and find the Labor senator ineligible because she failed to renounce her British citizenship in time for nomination.

The case is a test of Labor’s interpretation that candidates are not disqualified if they have taken “all steps reasonably required” to renounce foreign citizenship. Four MPs – Susan Lamb, Josh Wilson, Justine Keay and Rebekha Sharkie – have relied on the interpretation in refusing to resign.

Section 44(i) of the constitution prohibits a person who is a citizen or subject of a foreign power from election to federal parliament, a section that has resulted in the resignation or disqualification of nine MPs and senators in the 45th parliament.

Donaghue submitted that the court should apply the “natural and ordinary meaning” of the section – as it did in the Canavan citizenship seven case – and find that Gallagher was ineligible because she was still a British citizen at the time she nominated on 31 May 2016 and on the 2 July election.

With little interruption from the bench, Donaghue submitted that the only limit or exception to the general rule was where application of foreign law would “irremediably prevent” them taking part in Australian democracy, something that British law does not do.

He cited the fact that Gallagher, Malcolm Roberts, Fiona Nash, Jacqui Lambie and Skye Kakoschke-Moore had all succeeded in – eventually – renouncing British citizenship.

Gallagher’s counsel, the former solicitor general Justin Gleeson, submitted that in every case where a person had taken “all steps reasonably required” by foreign law – as he said Gallagher had – the exception of being “irremediably prevented” would apply.

That proposition was doubted from the bench. Justice Virginia Bell suggested it “denuded” the concept of being “irremediably” prevented and that framing the test in that way was “radically different” to the rationale for an exemption drawn from the earlier case of Sykes v Cleary.

Justice James Edelman suggested it created “almost a freestanding right” to run for office for a person who had taken all steps reasonably required.

Earlier, Donaghue submitted that Labor’s construction would allow people to be elected despite retaining the status, rights and duties of a foreign citizen. Picking up an observation from Justice Patrick Keane, Donaghue noted that was the case even where the foreign country was reasonable in exercising a power to refuse renunciation.

Gleeson defended Labor’s construction by arguing it had the “greatest simplicity, ease and certainty” and meant that people who had taken all reasonable steps would not find their eligibility at the mercy of foreign officials’ exercise of discretion or delay.

Justice Stephen Gageler suggested it was “a very odd way of looking at foreign law” that asked the high court to apply the steps in foreign law to renounce dual citizenship but then disregard the British law’s requirement that renunciation is not effective until it is registered. “It just doesn’t seem to work,” he said.

Earlier in an alternative argument, Donaghue submitted that Gallagher did not take all steps reasonably required because her renunciation was not submitted far enough in advance of nomination and she did not include all relevant documents until a further request from the UK Home Office.

After filling a casual vacancy in March 2015, Gallagher sent her application for renunciation on 20 April, 2016. The home office charged her the renunciation fee on 6 May, but then asked for further documents in a 1 July letter, which she received on 20 July. Her renunciation was registered by the home office on 16 August after she complied with the request by providing the originals of her and her father’s birth certificates and her parents’ marriage certificate.

Gleeson put particular weight on the date of 8 May, when the 2016 election was called, and said that by then Gallagher had taken “every step” required by British law for effective renunciation.

He said that she had provided the UK Home Office a correct declaration that she was a British citizen and “information showing” that in the form of the birthplace of her father on her own birth certificate.

Gleeson said the home office had an obligation to process the renunciation and submitted the request for further documents was not a requirement of UK law.

The court reserved its decision.