Not even a broken boot could stop Australia back-row forward Sean McMahon making an instant impression Sunday as he marked his Japan Rugby Top League debut with a try.

The 23-year-old Queenslander, who has already won 28 caps for the Wallabies, touched down in the 13th minute against Kobe Kobelco Steelers and set up a try for Matt Giteau as he played the first 40 minutes in a pair of modern lime-green boots.

Having punctured the front of one of those, McMahon then played a further 12 minutes in the second half in an old fashioned pair of black boots -- "that I always bring along in case something like this happens" -- before taking a well deserved rest.

"That was a lot quicker than I expected and was very physical as well," he said.

"Kobe are an exceptional side. But once I found my feet I had a couple of good ball carries. But I need to work on my game."

McMahon was originally supposed to join the club in May 2018, but a long-term injury to George Smith saw the club ask if their new signing could travel north for the back end of the 2017-18 season.

"They gave me a call and I just hope I can help them finish the year on a high," he said on the day Suntory found out they will play Yamaha Jubilo in the first round of the playoffs to determine the Top League champion.

McMahon said Smith's boots "were some of the largest to fill in international rugby." But with the backing of Suntory coach Keisuke Sawaki he has been asked to play his own game, rather than try to emulate the 37-year-old veteran, who played 111 times for Australia.

"Keisuke just said to get out there and be physical, carry the ball and do what I have been doing for most of my career."

McMahon said the club had been very welcoming in the week he has been with them and he was looking forward to the next two years.

"I can see why they are a championship side. You can see it in the way they train and play. There is a lot of pride in the jersey, the team and the company."