"The Prime Minister must sit in the House of Representatives. Can the Deputy Prime Minister be a senator?"

Hi Janet, thanks for your question about the role of Deputy Prime Minister.

As you stated, the Prime Minister is always a member of the House of Representatives. This is by convention (custom or tradition); there is no rule that the Prime Minister can not be a senator.

The Deputy Prime Minister is also always a member of the House of Representatives. This means that if the Prime Minister is absent or unable to work, the Deputy Prime Minister can act in their place. 

diagram to help explain The Prime Minister must sit in the House of Representatives. Can the Deputy Prime Minister be a senator?