The Abbott government approved Bob Day's potentially unconstitutional office deal against the advice of its own bureaucrats, who raised specific concerns about his financial stake in the building.

Government documents reveal the Department of Finance initially rejected Mr Day's request to relocate his electorate office from downtown Adelaide to a building he owned in the suburb of Kent Town.

But Mr Day - who resigned from the Senate this week after his national building empire collapsed - refused to take no for an answer and put the request to then-special minister of state Michael Ronaldson.

Mr Ronaldson sought a briefing from the department, which advised him in no uncertain terms to reject the proposal.