The Committee on Standards in Public Life is rewriting its guidelines on potential conflicts of interests.
It wants to know what the "reasonable limits" should be and whether they should be different for ministers and backbench MPs.
It comes after controversy over George Osborne's jobs in banking, public speaking and journalism.
The Conservative MP for Tatton, in Cheshire, will take up his new role as editor of the London Evening Standard on 2 May, according to sources at the newspaper.
Within 24 hours he will head off to Paris to address hedge fund managers and investors, according to the Press Association.
Standards committee chairman Lord Bew said: "We welcome views from the public and any interested parties on these issues.
"For example, what factors should be taken into account in determining the 'reasonable limits' on MPs' outside interests and is the current level of transparency sufficient?"
The consultation will run until Friday, 28 April.
The committee's findings of the review will be reported to the Commons Committee on Standards' review of the Code of Conduct for MPs in June.
An ethics watchdog want to know what the public thinks about MPs having second jobs.