I posted a link to the petition on Chris Bowen's FB profile. This was his response:
Dear Nicole. I completely understand your concern about this. To be granted a Prospective Marriage visa, the proposed marriage must comply with Australian law which means both partners must be at least 18 at the time of the wedding. There are also so other protections if the sponsor has a criminal record or there is evidence that the granting of a visa would not be in the best interests of the minor. Over the past 5 years 0.6 per cent of Prospective Marriage visas were granted to 17 year olds. This equates 207 people. Of those, the majority were marrying someone aged between 19 and 29. Around 3 per cent of all weddings in Australia involve women younger than 19 and where one of the people is from overseas the figure is pretty similar. The Department does conduct checks (which often involves interviews) to ensure every marriage is genuine and freely entered into. Thanks for getting in touch.