WENN -- 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 rumors of ill health, and now tour promoter Andrew McManus has opened up about Houston's breathing difficulties, pinpointing her tobacco habit as the cause.