5 Seconds of Summer plan to ''drunk sing'' at bandmate Michael Clifford's wedding.

The 23-year-old guitarist marked five years of dating his girlfriend Crystal Lee on January 11 by popping the question during a romantic evening watching the sun set while on vacation in Bali.

And Ashton Irwin and Luke Hemmings have told the hunk they can expect them to perform on his big day.

Asked his they have been asked to perform on the couple's big day, Ashton said: ''We haven't been asked yet!''

Speaking exclusively to BANG Showbiz, Luke said: ''I've made it very clear that I will be drunk and performing with him.''

However, Michael doesn't see how it will work without him.

He said: ''I don't know how you guys plan on performing.''

But Ashton reminded him that they performed a show drunk as a three-piece before at Capital's Summertime Ball.

He said: ''We've performed drunk once before at this exact show I think and we performed as a three-piece.''

Ashton then told Michael: :We can pull it off if you want that we should probably start rehearsing.''

Announcing his engagement, Michael wrote on Instagram: ''I was lucky enough to ask the love of my life to marry me in the place it all began for us.

''In the last 3 years she has helped evolve and shape who I am in ways I could never have imagined. I couldn't ask for anyone better to spend the rest of my life with. I love loving you, Crystal.(sic)''

He shared three pictures, including one of Crystal holding her face in delight with her ring on display, another of them lying down together and a third of him down on one knee popping the question.

The romantic musician took Crystal to the Ungasan Clifftop Resort in Bali, the same spot where they had shared their first kiss and set up a tent outside their villa, which he filled with photos of them during key moments in their relationship, including their first picture together, their first 'I love you', and their first anniversary, and even played 'Medicine' by The 1975 because it was what they had been listening to when they locked lips for the first time.

Later in the evening, Australian chef James Viles cooked a meal for the couple and then they enjoyed a celebration with some friends, who were also on the trip because it was a promotional event for Guess' #DestinationGuess Bali series.