Apartment building is set to to collapse 50 per cent over the next four years, according to forecaster BIS Shrapnel, with only Sydney set to buck the trend.

Its Building in Australia 2016-2031 report says national dwelling commencements have already peaked and will begin to decline in the second half of this year.

"After recording strong growth during the past four years, we estimate that total dwelling starts reached an improbable 220,100 in 2015-16, an all-time high," said BIS Shrapnel associate director Kim Hawtrey.

"From this level, national activity is forecast to begin trending down over the following three years, with the high-flying apartments sector leading the way down."