Hrustic found the back of net in the 87th minute of the Eredivisie clash after running on to a deft ball over the top before chipping the keeper for the match winner.
It was the perfect start to the season for the 23-year-old who has had mixed fortunes since breaking onto the scene as a 20-year-old with a shock call-up from then Socceroos coach Ange Postecoglou for Australia's match against Brazil in 2017.
The win moves Groningen up to third spot on the ladder.
It was a great matchday as well for another Socceroos player with defensive stalwart Trent Sainsbury given a rare start for PSV Eindhoven.
Sainsbury started the match and played the full 90 minutes in the 1-1 draw away at FC Twente.
The 27-year-old will be hoping the match is the beginning of more regular action in the first team after a horror first year in Holland where he featured in only nine games.
In England, the start of the Championship meant it was back to action for a number of Socceroos regulars.
Jackson Irvine played the full match as Hull City suffered a 2-1 defeat away to Swansea while Milligan was also on the wrong side of a losing scoreline going down 1-0 to Coventry City in his debut for League One outfit Southend United.
Scottish-born Socceroos bolter Harry Souttar had a better start to the League One season scoring for Fleetwood Town in their 3-1 win away from home at Peterborough.
Souttar started the match in the centre of defence and played the whole 90 minutes.
Ryan Edwards also saw action in League One playing 90 minutes for Burton Albion in their 1-0 loss to Ipswich Town.
In Austria, Jimmy Jeggo faced off against fellow Aussie James Holland as his Austria Vienna slumped to a 3-0 defeat at LASK Linz. Holland and Jeggo played the full match for their respective teams in the one-sided result.
Finally it was a frustrating 1-0 defeat for the Ange Postecoglou coached Yokohama F Marinos.
Postecoglou's side dominated proceedings against Shimzu S Pulse finishing the match with 80 percent possession and 586 more completed passes
However, it wasn't enough as Shimzu snatched victory after bundling in a solitary goal in the 50th minute and hanging on to collect all three points.