I signed a legal document as guarantor, the document had its…

I signed the original credit application guarantee when my brother was purchasing a new smash shop and it was for paint but that business purchase fell through. I was told by my Brother that credit application was cancelled as it specific for that business & that we would need to reapply. My Brother & the paint supplier had a meeting where my Brother altered the credit application by whiting out the old business details & added the new business details without my knowledge. He was told that this would not be acceptable but later when I withdrew from my Directorship of my brothers company the paint supplier has tried to hold me liable for the $50k debt by using this altered credit application document.

you are correct, I signed as director of a holding company purchasing a smash repair with the credit application under the smash repair my Brother was purchasing. that deal fell through & we were told that the credit application was cancelled. When he went to purchase the new smash repair shop he altered the old credit application details with my guarantor signature on it & without my knowledge. He was told that this was unacceptable at the meeting but then a few weeks later a letter was sent saying that the account was open. I wrongly assumed that my Brother had filled in a new credit application without my guarantee.
I withdrew from as the director & was sent a letter from the paint supplier stating I owed them $50k & that would sue me for the amount as I am the guarantor.They have produced the altered document as proof of my guarantee but you can clearly see that is has been altered. My Brother is willing to sign a statutory declaration stating that I did not know of the alteration but the paint suppliers lawyer is stating that they accepted this credit application with my guarantee under "good faith" & that I still am the guarantor.
I have a photo copy of the altered document & you can see the old details at the bottom

Hi Leon, I have done so & even offered $5k to offset their legal costs but they say I am liable for the fully $50k plus legal costs. is there any points that you think nulls my liability ?
I believe that due to the document being altered without my knowledge that I would not be liable ?
Their legal states that I am as under NSW law they accepted this credit application with my guarantee under "good faith" & that I still am the guarantor hence why they are pursuing me.

Hi Leon, the company that I was a director of did not change but the business it purchased did hence the altered document which whited out the original business to be purchased & put the eventual business purchased details. No finance was provided but a credit account in which goods (paint) was supplied to the value of apprx $50k. I was the director of the holding company but did not operate the business & resigned as director once the business was established. The paint company is pursuing me as the know my Brother is unable to pay. My question is .. is there a precedent of law in NSW that can be quoted as the major sticking point is that they are holding my guarantee on an altered document which was altered without my knowledge or approval this does not seem right...