Google policies are clear on this point : it’s forbidden for companies to post promotions, contests or sweepstakes directly on their Google+ Page.
Instead it’s possible to create a link to the page that contains the contest or special offer, as long as it’s allowed in the country of the owner of the page.
Google warns that they will delete or block pages that are not in compliance with the regulations. Ok that’s a good point to prevent spam, but with this policy, the Google+ Page is not as useful as it could be for a brand…
Solution: link directly to a page of the blog/corporate website, to a Facebook page or a Twitter account (it’s allowed).
Details of the rules & policies for a contest or promotion on a Google Plus Page : http://www.google.com/intl/en/+/policy/pagescontestpolicy.html
Ps: The picture above is Humor!