Multiple text messages from China - spam

I am in Australia and have been getting masses of text messages all at once from various numbers all in China, the text is in Chinese characters. It happened today and I think I have over 100 text messages that I have to individually go through and block, which takes ages. Also happening a few days later is I get a pop-up notification that my iMessage is being used on some other device - a name with Chinese characters. I am assuming these 2 events are linked. I have repeatedly changed Apple ID passwords but it still happens. My phone provider (Vodafone) cannot block these text messages and I’m concerned what all this means securitywise.

I went into my Apple store and showed them. They didn’t know what the problem was and advised me to change my email password and Facebook password, which I did, but this is still happening.

Any help appreciated.

thanks