Double dissolution or just the lower chamber?
Almost certainly a double dissolution if he decides to go. The government has firm control of the House, but the balance of power in the Senate is in the hands of the Greens and a bunch of tiny party/independent Senators, and that's why the government is having a very hard time getting legislation passed. A normal half Senate election isn't constitutionally possible until July next year (and Senators elected then would not take office until July 2017) so the only way to change the Senate is a double dissolution.
The problem with that is, that the reason we are in this mess at the moment is because at the last election, people voted for micro party candidates in the Senate in numbers that have never been seen before, meaning the current Senate is totally different in make up to any previous one, and in a full Senate election (from a double dissolution) the quota for election is halved, making it even more likely micros and independents would be elected. Even if we retained the House, we might end up with a worse situation in the Senate. If not for this new factor (which really comes down to the internet making it easier for people to work out how to vote 'below the line' for a micro-party or independent than it has historically been), I'd regard a double dissolution as likely - but it changes things so much that I think it's actually quite unlikely unless Labor truly implodes (if Bill Shorten was found to have acted corruptly as a union official and refused to resign, it could do it - but I honestly hope he didn't, I don't agree with his politics, but I'd like to think he is honest - at least honest for a politician).