Thanks for your question. The parliamentarians moved from Old Parliament House to New Parliament House because there was not enough space in Old Parliament House.
When Old Parliament House opened on 9 May 1927, there were 101 members of parliament—by the time the building closed, this number had more than doubled to 224.
Despite many additions and alterations, by 1988 the provisional building was completely inadequate for the needs of the contemporary Parliament. About 3000 people were crammed into a space originally designed for several hundred.