Already Australia's all-time leading scorer with 42 goals before Tuesday's Asian Group B clash, the evergreen Cahill took his career tally to 45 with three first-half strikes.
The Socceroos captain Mile Jedinak also found the net as the reigning Asian champions cruised to 4-0 lead by halftime then took their foot off the gas against a Bangladesh team ranked 120 places below them.
The Australian coach Ange Postecoglou said his team had not performed at their best but there were some extenuating circumstances because of security-related issues in the lead-up to the match.
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Last month, Australia's world champion cricket team postponed their tour of Bangladesh after the Australian government advised them to stay at home because of potential militant attacks.
"Knowing what the boys have been through the last few days, both physically and mentally, we lost our focus and concentration in the second half," Postecoglou told Fox Sports.
"It became very scrappy. When that happens it's not about goals, we just didn't maintain our composure.
"But with everything that's going on, the players, the staff have handled themselves impeccably and pleased we can get through unscathed."
Australia's resounding win took them to the top of the standings with 15 points from six matches with their remaining two games at home in March.
Jordan are second, two points behind the Socceroos, after losing 1-0 in Kyrgyzstan later on Tuesday.
The eight group winners and four best runners-up will all progress to the next phase, which decide Asia's four automatic qualifiers for Russia in 2018.
Bottom-placed Bangladesh, with one draw from seven matches, are out of contention, along with Tajikistan, the other team in Pool B.
Cahill's only previous hat-trick for his country was against Fiji in 2004, the same year he made his international debut. The 35-year-old had scored two goals in a match on eight occasions but his long wait for another treble ended quickly on Tuesday.
Cahill opened the scoring after six minutes when he headed in a free-kick from the impressive Aaron Mooy. He made it 2-0 after 32 minutes when he volleyed in from close range then 3-0 when he slotted into empty net after Mooy broke free.
Mooy also laid on Australia's final goal, just before the halftime break, when the midfielder's free kick into the box was nodded back from Bailey Wright to Jedinak for the easiest of tap ins.
"It was a great result, it's been difficult circumstances but overall I'm disappointed with the second half," Cahill said.
"I felt we could have done a lot better if we had stuck to the game plan and been ruthless.
"In these games you come away to difficult places to get the right result but it's always great to score goals because that's what I do."
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