Holbrook, 43, who joined Saints in 2017, has guided them to a first Challenge Cup final in 11 years after they overcame part-time Championship side Halifax on Saturday.
The Merseyside club are also 10 points clear at the top of the Super League.
Holbrook is expected to see out the rest of this season with Saints.
The Australian’s future was left in his own hands by the club, with chief executive Mike Rush recently saying they “don’t need to sell him a dream” and admitting that Holbrook’s success in England made him a target for NRL sides.
Last season Holbrook guided St Helens to the League Leaders’ Shield, although their hunt for major silverware ended with semi-final exits in both the Super League play-offs and Challenge Cup.
They are again on course to top the table at the end of the regular season and have already gone one better in reaching the Challenge Cup final, where they face nearest Super League rivals Warrington on 24 August.
Holbrook will take over a struggling Gold Coast side that sacked Garth Brennan earlier in July with the club bottom of the NRL.
His last job in the southern hemisphere competition was with the Sydney Roosters, where he was part of the coaching team that won the NRL minor premiership in 2015.
He also previously worked at Canterbury, St George and Parramatta and was in charge of the Australia Under-20 side.