Whitney Houston's tour promoter has offered an explanation for the singer's faltering voice - the star refuses to quit smoking.

The R&B superstar took her Nothing But Love shows to Australia this month, but the tour has been marred by complaints from critics and fans Down Under who claim she has been struggling to complete her sets.

Houston allegedly coughed her way through a show in Brisbane, taking regular breaks to catch her breath, while fans are reported to have walked out of her concert in Sydney when the singer's voice began to crack.

Her spokesperson recently shrugged off rumours of ill health, and now tour promoter Andrew McManus has opened up about Houston's breathing difficulties, pinpointing her tobacco habit as the cause.

McManus explains, "Whitney smokes. God bless her, I can't tell her not to. She will not take any substances. She just will not. It is what it is. She's comfortable with the show she's performing. She knows she's not hitting every note like she used to. But it's value for money, it's Whitney Houston warts and all."

And he admits Houston is relying on her Christian faith to help her finish the road show, adding, "I talked to her after the Sydney show. I told her I have her back but she said, 'If the good Lord wants me to be attacked and ridiculed that's the road I have to go down.'

"It's an interesting approach. Here I am being aggressive in trying to protect her and she said, 'I love you for it but let it go. I can only give my best, if that's not good enough, so be it. I'm comfortable in my own skin.'"