Robert Irwin may have gotten himself in more hot water after describing his pregnant sister as "defying gravity."

The 17-year-old and his mother Terri were asked how Bindi was going at a funding announcement on Saturday morning at Australia Zoo.

After saying he was going to "choose his words very carefully" Robert continued to dig himself a deeper hole following comments on national television two weeks ago where he copped a cheeky punch on the arm from mum after describing Bindi as "massive."

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Robert Irwin said his 22-year-old sister was doing great in the final stages of her pregnancy.

"She is defying gravity with that belly and it's really exciting," he said.

Mum Terri chimed in and said "and we're back" with a laugh.

Robert sheepishly continued, "anyway what I mean is she's doing good, the baby is doing very well."

"It's going to be imminent, they reckon that probably the end of around next month is when she's due.

"But the baby is apparently all ready to go so it could be soon.

"We're all very excited.

"It's all locked and loaded."

Terri said she thought Robert could have been an obstetrician with his assessments of his big sister's pregnancy.

While the family remained coy on potential baby names they did reveal their nicknames.

"We have our names though I'm being called the 'funcle' the fun uncle and mum is going to be grandma bunny," Robert said.

"Bunny is actually, in America and Europe, it's an animal nickname for a grandma.

"So that's going to be our nicknames, no names yet for the little baby but we'll see."