Terri Irwin's mother Julie Raines said yesterday Steve Irwin's shock death had been "very hard" on his family.

Raines said her two-yearold grandson Robert kept asking his mother "where's Daddy?"

But Raines said her eightyear-old granddaughter Bindi, who has made regular TV appearances with her father, had been "a rock" for Terri.

Irwin, 44, died on Monday after he was pierced in the heart by the barb of a stingray while filming a documentary off Port Douglas in north Queensland, Australia.

His mother-in-law, who lives in Oregon in the United States, said Terri Irwin had told her in a telephone conversation about how she was struggling to cope after his death.

"She said 'I'm concerned about raising the children by myself ', " Raines said in a telephone interview. "But I know she'll do well, she always does. She's just having a hard time. Robert says 'where's Daddy?' Bindi's been a big help to her. Bindi's been a rock. It's just a very hard time."

Mourners continue to pay their respects to Steve Irwin at the family's Australia Zoo north of Brisbane as preparations continue for a private funeral.

The Irwins have agreed to a public memorial service, possibly at Brisbane's Suncorp Stadium, in the next few weeks.

Irwin's friend and business manager, John Stainton, said planning for the memorial service would be started once the funeral was over. He said Terri Irwin was comforting her children and did not want to speak to the media.

Steve and Terri met in 1991 when the American-born zoologist visited a zoo where he was working. They had a whirlwind romance and were engaged a few months later.

They were married on 4 June 1992, and while on their honeymoon, they filmed their first documentary together.

It later was used as the first episode of the Crocodile Hunter series which has made Steve Irwin famous around the world.