The Steelers beat the Sagamihara Dynaboars 59-33 on Saturday to lock up fifth spot on the standings.
Rennie has presided over steady improvement at the Kansai-based club since he joined two years ago, with Saturday's success earning Kobe a shot at the post-season for the first time since their last title seven years ago.
One of the teams standing in Kobe's way is also coached by a former Wallaby boss in Robbie Deans, whose Saitama Wild Knights remain top of the table despite a valiant effort from Black Rams Tokyo, with All Black scrumhalf TJ Perenara scoring two tries in their 27-21 loss.
Spears Funabashi Bay, who play the Wild Knights in a box-office clash next weekend, remain second after coming from behind at halftime to beat Mie Heat 39-20.
Springbok hooker Malcolm Marx scored twice, after former Wallaby flyhalf Bernard Foley scored the Spear's opening try.
Kobe's likely opponents in the playoffs, Shizuoka BlueRevs, were led by another length-of-the-field try from former NRL winger Valynce Te Whare, as they came back from 0-21 down to beat Yokohama Eagles 38-28 on Sunday.
The 24-year-old's sixth try of the season, and second in three weeks, was part of a period where the BlueRevs scored 31 unanswered points to seal a win that could have ended the chances of Wallaby Mathew Philips' Eagles making the playoffs.
Yokohama trail Tokyo Sungoliath by six points with two games remaining after ex-All Black skipper Sam Cane scored twice in Sunday's 45-28 win over Verblitz.
Steve Hansen's side remain in relegation peril ahead of next week's clash with the Heat, who are one place below, with the losers likely to join Urayasu D-Rocks in the end-of-season promotion/relegation series.
Samu Kerevi's ninth try from his past eight appearances couldn't prevent D-Rocks slumping to their 13th defeat of a wretched season, after the league's bottom-placed side were outclassed 61-19 by champions Brave Lupus Tokyo on Friday night.