Kim Dotcom will tonight reveal what is claimed to be a "bombshell" on New Zealand's involvement in spying.

Kim Dotcom speaks to ONE News. Source: 1 NEWS

Now just five days out from the election, Mr Dotcom says he will reveal at Auckland Town Hall "what John Key, the National Government and Hollywood don't want you to know".

He will be joined by Pulitzer prize-winning journalist Glenn Greenwald, who published NSA whistleblower Edward Snowden's evidence of mass surveillance in the US.

Tonight's event will be live streamed on onenews.co.nz from 7pm

Mr Greenwald told TV ONE's Q & A programme he has proof the GCSB has been conducting mass surveillance and that Prime Minister John Key has been deceiving the public about it.

The Prime Minister completely denies the accusations, and speaking on Q & A yesterday said: "Without a shadow of a doubt GCSB does not undertake mass surveillance against New Zealanders nor have they."

Mr Key says he will release documents showing spy officials may have considered mass surveillance but the proposal never went ahead.

However, Mr Greenwald says "there are documents that call into serious question the truthfulness of those claims".

"The GCSB engages in mass surveillance of exactly the type they denied to the public.

"And we've been spending months doing the reporting and are very close to divulging the reporting that I think will let New Zealand citizens decide whether [the claims] were truthful."

Mr Dotcom and Mr Greenwald will be joined by Wikileaks founder Julian Assange, likely to be via video link, and international human rights lawyer Robert Amsterdam.

ONE News Deputy Political Editor Mike Parkin says he feels the impact of the revelations will be limited, and is unlikely to sway voters.

"I don't think there's anything in here that will be enough to have the Prime Minister resign tomorrow, I think even Dotcom's backed away from that kind of line," he says.