* Australian High Court to rule on fate of seven lawmakers
* Ruling to determine whether government can keep majority
* Australian government argues five should be cleared (Adds details of government's legal defence, deputy PM's submission, paragraphs 6-7, 10-11)
By Colin Packham
CANBERRA, Oct 10 (Reuters) - Australia's High Court began a three-day hearing on Tuesday to determine the eligibility of seven lawmakers who may hold dual citizenship, a case that could determine whether the government is able to maintain its razor-thin majority.
Australia's parliament was rocked by revelations in August that the seven politicians, including the deputy prime minister and two other Cabinet members in Prime Minister Malcolm Turnbull's coalition government, are dual citizens. means they are potentially ineligible to hold elected office under Australia's constitution. unpopular centre-right government holds just a one-seat majority in parliament, meaning its future could rest on the outcome of the citizenship crisis.
The seven lawmakers accept they were dual nationals at the time of their election last year but the government argues that five of them, including all three Cabinet members, should be cleared because they were unaware that they had contravened the constitutional requirement at the time.
Australian Solicitor-General Stephen Donaghue urged the seven justices of the High Court not to interpret the constitution literally.
Donaghue said instead the constitution should only disqualify politicians if they had prior knowledge that they may be dual citizens but did not take "reasonable steps" to investigate and renounce their second citizenship where needed.
A ruling could come as soon as Thursday.
Should the High Court rule Deputy Prime Minister Barnaby Joyce, the sole lower house lawmaker caught up in the crisis, is ineligible, Turnbull will then need to win the support of one of three independent lawmakers to keep his minority government.
Joyce acquired dual citizenship from his New Zealand-born father. He said in a submission to the court he believed that his father had renounced his New Zealand citizenship before he was born.
He did not know until he was 10 that his father was still a New Zealand citizen, Joyce said.
Turnbull's government is already in a minority in the upper house Senate and, should the High Court disqualify all seven lawmakers, he will face a government reshuffle after losing two more Cabinet members who are senators.
Support for Turnbull continues to languish near a six-month low, but political analysts believe he might receive a boost if he is able to win passage of a same-sex marriage bill.
Turnbull's government won High Court approval last month to stage a non-binding postal survey on the issue, which is widely popular among Australians. Results of that poll will be announced on Nov. 15.