The world is full of mysteries. Why did the crew of the Mary Celeste abandon the ship? Do big cats reside in parts of Victoria and NSW? What do people see in the music of Nickelback?

And here's another one — what's become of the tax refunds deposited in the bank accounts of millions of Australians since the start of the new financial year?

Have Scott Morrison's tax refunds flowed into retail spending?

The low- and middle-income tax offset, the centrepiece of the Josh Frydenberg's first budget, promised people earning between $50,000 and almost $90,000 tax relief worth $1080. Millions of others would also get some respite.

It was the first part of a three-year plan that the government argues is the biggest tax cut in two decades (even though they will not be fully in place until 2024-25).