“I’ll Have a Think About It” Means “I’ll Consider It”

“I’ll have a think about it” means that you will consider something and answer later. It is entirely natural in everyday Australian conversation. The phrase does not signal agreement; it simply postpones the decision. Its tone is usually calm and less formal than “I need time to consider”. If a friend suggests a weekend trip, you might say, “Sounds good. I’ll have a think and let you know.” Chinese speakers sometimes use “I will consider”, which is grammatical but can sound rather formal in casual conversation. “Have a think” is common in Australian and British English, while American speakers are more likely to say “I’ll think about it”. The phrase creates a small amount of space before an answer.

https://dictionary.cambridge.org/dictionary/english/think
https://www.abc.net.au/education/learn-english/how-to-talk-about-your-mental-health-feelings-emotions/13562670