The SIS, in an effort to claim that Friday's terrorist attack which killed fifty people wasn't a failure on their part, is saying they had their eye on far-right extremists. It's an extraordinary claim, in contradiction to everything we know about their past behaviour and priorities. Fortunately, its easy to prove.
Since 2002, the SIS has been granted 314 intelligence warrants. These are basicly the sign of a serious investigation, allowing phone taps, computer hacking, and black bag jobs to gather evidence of interest to "national security". If they've been properly keeping their eye on Nazis, then there will be intelligence warrants for them. All they have to do to prove they've been doing their job is tell us how many there are, per year (or per 3 years, to give some statistical cover), relating to that sort of target.
They won't want to do this, of course. Telling us even broadly who they are spying on is anathema to the SIS. But they've already kindof done that by saying they've been surveilling Nazis. If they want us to actually believe it, and believe the level of surveillance was appropriate to the threat, they need to provide some proof. They need to show us the warrants.