This is a big deal because it threatens the government majority

There have been resignations from parliament over senators holding dual citizenship

This is a no-no under the law here if you are an MP

The resignations have come from minor parties, not threatening the government

This time though ...

  • Liberal MP Julia Banks may be a dual Greek-Australian citizen and ineligible to sit in Parliament

Banks is a government MP, if she goes there will be a by-election in her seat, its loss (if that was the outcome) could threaten the government hold on power (the Turnbull government holds power in the lower house by just one seat)

(ps. I wouldn't be too worried about all this, whatever the outcome in Australia it's a Tweedledum or Tweedledumber choice for PM. Still, it might induce a wobble or two).