The fast bowler has put pen to paper on a deal that will see him play the first six matches of the LV= Insurance County Championship Division One campaign.
Northants open up against Gloucestershire at Wantage Road on April 7, and Kelly will be in the County ranks for that fixture.
The 27-year-old will then be available for trips to Essex, Surrey and reigning champions Warwickshire, as well as home encounters against Yorkshire and Kent, which is scheduled to be his last game for the club on May 19.
Kelly has been a regular in Western Australia's Sheffield Shield line-up for the past four years, and has enjoyed genuine success in his home country, claiming more than 100 wickets at an average just shy of 30 apiece.
Those stats have been achieved using a Kookaburra ball on hard, flat surfaces, and Liddle feels Kelly is a player who could be even more effective in English conditions, especially with the Dukes ball in matches played in April and May.
“I’m delighted to have Matt join us for the first few months of the summer and really excited to see him get a Northamptonshire shirt on and get going out there for us," said Liddle, who is also the County's bowling coach.
“Matt possesses a fantastic skillset that we feel is well suited to English conditions and will complement our bowling attack nicely.
“He’s got a great record in red ball cricket and we think he’s a perfect fit for what we need.
"I can’t wait to get him over here and see what he can bring to the group.”
Kelly will be joining an already strong seam bowling unit at the County Ground, and his arrival ensures competition is going to be fierce among the likes of Ben Sanderson, Gareth Berg, Tom Taylor, Brandon Glover, Nathan Buck, Jack White and Luke Procter.
The Claremont-born bowler becomes the third overseas signing made by Northamptonshire so far.
He joins New Zealand batsman Will Young, who has been signed for the entire red ball campaign as well as the Royal London One Day Cup, and Kiwi all-rounder Jimmy Neesham who has been snapped up for the Vitality T20 Blast.
The club will still be looking to add a further signing to act as back up to Neesham in the Blast, and Kelly is somebody who is a more than handy white ball performer.
His record in T20 and 50-over cricket for Perth Scorchers in the Big Bash and WA respectively is more than decent.