Hi Liliana! There are no rules about a shadow minister sitting in the same house as a minister. For example, currently the Minister for Defence is a senator but the Shadow Minister for Defence is a member of the House of Representatives.
If a minister is a member of the House of Representatives, a member of the ministry in the Senate will answer questions and introduce bills relating to the minister’s portfolio (area of responsibility) in the Senate. The same arrangement is used for ministers who are senators.
Money bills cannot be introduced into the Senate (Section 53 of the Australian Constitution), only into the House of Representatives. If the Treasurer was a senator, their representative in the House of Representatives would have to introduce the budget, not the Treasurer.