A deadly terror attack at Sydney's Bondi Beach is feeding speculation of potential intelligence errors. 

Two gunmen opened fire at a Hanukkah celebration on Sunday evening, killing 15 people and wounding dozens more. 

A 24-year-old suspect remains in hospital under police guard - while his 50-year-old father, the other alleged offender, was shot dead. 

The younger man, who's suspected of orchestrating the attack, had previously been investigated by the country's security agency - and hadn't been csydnonsidered a threat. 

But Massey University Defence Studies Professor John Battersby says we shouldn't jump to conclusions - until more details emerge. 

He told Andrew Dickens intelligence capabilities are always limited by the resources put into them.