Colleague returns from suspension to give silent treatment after I reported him for taking credit for my work

I'm in Victoria, Australia, and I need advice about a workplace situation that's escalating into social isolation and potential career damage.

Liam came back from a two-week suspension yesterday, and he's giving me the full silent treatment. He won't look at me, won't acknowledge me in the kitchen, and he's actively encouraging others to do the same. I've seen him whispering to people and then glancing at me. It feels like I'm back in high school, except my mortgage depends on this job.

Priya, who was in the 3 March meeting when Liam presented my data as his own, has gone completely quiet. She's not returning my calls or texts. I know she saw what happened – she was sitting two chairs away and even asked Liam a question about the data that made him stumble. Now I'm wondering if she's scared of Liam too, or if she's decided it's easier to just let me hang.

I found out yesterday from a friend in another team that Liam has been telling people I'm a 'troublemaker' who lies to get ahead. He's saying I fabricated the whole thing to get him in trouble. Worse, my manager mentioned to me this morning that there's talk of putting me on a performance improvement plan. I've never had any performance issues before this.

So I emailed HR requesting a formal mediation session. I attached a timeline of events, including screenshots of the meeting agenda where my name was removed from the data slide, and I named Priya as a witness. I know that's a big step, but I feel like I have to protect myself.

Priya finally got back to me tonight. She admitted she saw everything – Liam reading from my spreadsheet, presenting my analysis as his own – but she was scared. She said Liam has a reputation for ruining people who cross him, and she didn't want to be next. She's agreed to support me in the mediation.

My question is: what are my rights here? Can my manager put me on a PIP based on a rumour when I have a witness? And what protections do I have against this kind of social bullying at work? Should I talk to a lawyer?