The United Workers Union has written to clearing and security contractors that look after buildings including the Royal Australian Mint in Deakin, the Australian National University, Old Parliament House, Defence Housing Australia and Questacon, asking them to pay their workers when the buildings are closed.

Many were closed on Sunday and Monday. All have been open since but could close again if and when thick smoke returns.

So far, only Rolfe and Kamberra, which clean old Parliament House, Defence Housing Australia and Questacon, had committed to paying staff according to the roster.

A union spokeswoman said cleaners and security contractors employed people through full-time, permanent part time and it was easy for casuals, especially, to get left behind.

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If workers were rostered, employers should do the right thing and pay them, even when buildings were closed, she said.

"We have asked for a commitment that all rostered staff will be paid for their rostered hours when buildings have been closed," she said.

"All workers in affected buildings must be extended the same rights to be paid. Low-paid workers mustn't be left behind.

"Closures and unpredictable events can be particularly harsh for casual workers, who still have to pay the bills and meet the costs of living.

"Permanently employed public servants in these buildings know they will be paid across closure periods, and it should be the same for all workers no matter their job in the building or venue," the union said.

Security guards had reported to their union that smoke had made them unwell and some hadn't been able to work.

While Canberra has had a relatively clear week, the smoke haze is forecast to continue.

The Bureau of Meteorology is forecasting possibly thick smoke on Saturday and Sunday.