Do visa applications to China often get declined?

I am a New Zealand citizen and my girlfriend is a malaysian citizen with permanent residence in New Zealand. We have an invitation letter from my aunty in China however we plan to travel to China from Hong Kong via ferry and so we don't have any flight itineraries. We are worried that we will not get a visa in time to go to China. From your experience, is it difficult to obtain one from Bangkok? We also have no criminal records or anything like that.

Thanks in advance!