A Turnbull government cabinet minister has been caught out failing to declare her financial interests properly, potentially placing her in "serious contempt" of Federal Parliament.

Employment Minister Michaelia Cash did not declare a mortgage on a $1.4 million investment property for almost four months - and finally made the disclosure only after questions from Fairfax Media.

Senator Cash bought the house next door to her home in the upmarket Perth suburb of Floreat on November 4, property records show.

It's the fourth house in her portfolio, and the second investment property. Senator Cash took almost three months to declare the house but then did not declare the new mortgage - both of which are required under the rules.