Australian sweetheart Lisa McCune, who shot to fame at age 22 as Constable Maggie Doyle in Blue Heelers, is about to take on her first middle-age roles. And it feels like “the right fit”, she says.

Best known for her television work and, more recently, her roles in musicals such as South Pacific and The King And I, McCune returns to straight theatre in a new Australian play called Machu Picchu opening tonight in Sydney in which she plays Gabby, a middle-aged civil engineer, whose life and marriage is upended when her husband is involved in a serious car crash.

In May, she plays Sally, a former showgirl who is now 49 but “still remarkably like the girl she was thirty years ago” in a concert version of the Stephen Sondheim musical Follies in Melbourne.

McCune, who recently turned 45, still looks extremely youthful herself but is very happy to embrace the more mature roles coming her way.

She describes Machu Picchu as: “a grown-up play about relationships and mid-life” and “a fascinating subject matter.

“It’s my first role playing (a middle-aged woman). Well, I suppose Anna in The King and I was too. But it’s a more mature part of my life as well so that is interesting to me. It’s not the ingénue any more. And it’s really nice to explore that,” says McCune.

“I don’t consciously go ‘oh these characters are older’. I think it’s just that feeling of the right thing. I certainly not going to be botoxing to try and play ingénues who are in their late 20s. It’s just where you start to fit I think.”

Though she denies giving too much thought to getting older, McCune admits that she felt different the second time she played the vivacious nurse Nellie Forbush in South Pacific.

“It’s funny, I did the first season of South Pacific (in 2012). By the time I came back to it a year later, I thought ‘I can’t play this any more’. I’m actually getting too old to play her and that’s when I did Anna and that felt right.

“It’s nice with this play (Machu Picchu) that it hasn’t been done before and there are no comparisons. We’re starting from scratch.”

Machu Picchu is a co-production between Sydney Theatre Company (who commissioned it) and the State Theatre Company of South Australia. It is written by Sue Smith, whose TV credits include Brides of Christ and Mabo and whose last play Kryptonite was premiered by STC in 2014.

It centres on a middle-aged couple who seem to have it all, but who must reassess their lives, priorities and relationship when the husband Paul is left a paraplegic after a car crash. The play moves back and forth in time, so that we see them before and after the life-changing accident.

Machu Picchu is worlds removed from the lavish musicals McCune has performed in of late, throwing up many questions about how you live your life after such an event. McCune’s character also wrestles with a lot of guilt.

McCune agrees it’s “tricky subject matter” but says the play has “many humorous moments” and “great heart. I think Sue’s language is really beautiful and her observations are fantastic,” she says.

In rehearsals, McCune found that performing in a straight play has required her to flex different acting muscles.

“It sharpens different things and that’s great. It’s the different language. A musical heightens things in different spots whereas when its just text, not backed up by songs, it doesn’t happen that way. So for me, it’s a new experience again. I’m finding it challenging – which is what you want out of your work really.”

Darren Gilshenan, who plays Paul, believes the role is very different to the parts McCune is usually cast in.

“I think she’s really excited that people will see her in a different guise and see what she’s capable of. This allows her to dig deep and find a lot of ugliness as well that you wouldn’t normally associate with Lisa,” he says.

Gilshenan is a fine comedy actor whose numerous credits include the seemingly hapless but kindly, occasionally wily neighbour Jack in Channel Nine’s Here Come the Habibs!, Uncle Terry in ABC-TV’s The Moodys and Bell Shakespeare’s hilarious production of The Servant of Two Masters.

McCune and Gilshenan have worked together once before: in the musical Urinetown for STC in 2006.

“I’ve admired his work for a long time. He was one of the reasons I wanted to do (Machu Picchu),” says McCune.

On the face of it, it seems almost perverse to cast an actor renowned for his physicality as a character in a wheelchair. But Gilshenan’s physical skills will be tested in Machu Picchu. “The level of detail that happens in the nine months (Paul is) in hospital, his development physically through rehab and the various stages needs to be very clear,” he says.

“I was saying to my wife after the first couple of day’s rehearsal that the scope of this piece emotionally, physically and intellectually, and what I can do with it, is fantastic. It’s a dream role.

“A lot of the comedy I’ve done recently is based on humour through the pain and truth of flawed individuals. But there’s always an awareness you’re in a comedy, whereas in this it’s really surprising where the real tragedy is at times. In the awfulness of the situation, there also a lightness and a comedy in there too.”

Smith has described the play as “both a grand love story, and a deeply ordinary one”, as well as a celebration of courage.

“That’s the part I love about it,” says McCune. “I’m such a romantic. I think the romantic side of it is really beautiful. It’s about a couple who are somehow meant to be together and how they are going to travel the next part of their life that’s really hard. It’s a tricky subject matter. I think Darren has had a lot to contemplate for his role.”

Asked if anything has happened in her own life to make her stop and take stock, McCune says: “I think for me the biggest turning point in my life was having children.”

McCune, who has never commented on her reputed relationship with opera singer Teddy Tahu Rhodes, has three children aged 15, 13 and 11 with her husband Tim Disney.

Machu Picchu rehearsed in Adelaide and has a season there after its Sydney run – which means a fair amount of time away from her Melbourne-based family.

“It’s one of the lines in the play: ‘work is work and it must be attended to,’” says McCune.

“It’s a different part of my life. Isn’t it funny: that’s what the play is about. It’s about living your life. And I guess for me I’m a better mum when I am doing some work. I think I feel more content. I’m happier and I’m happy to go and really throw myself back my life at home. I kind of need it.

“The kids are fantastic. They are a little bit older now and they understand that that’s the life that we’ve chosen (and) that they’re a part of. So they are OK and we just make sure we talk about it with them. And they’ll come away for holidays. Once you start throwing in holidays, you’re not actually away for that much.”

Machu Picchu plays at Wharf 1, Sydney until April 9. Bookings: 9250 1777. Dunstan Playhouse, Adelaide Festival Centre, April 13 – May 1. Bookings: BASS 131 246

 A version of this story ran in the Sunday Telegraph on March 6