The growth in new coronavirus cases in Australia is slowing but experts say there is still a long way to go in controlling the spread of Covid-19.

On Sunday and Monday, the federal chief medical officer, Prof Brendan Murphy, and the New South Wales chief health officer, Dr Kerry Chant, both expressed qualified optimism about the numbers of new Covid-19 cases being reported over the previous days. The charts showed good signs, they said, but there was a long way to go.

Analysis of coronavirus case data compiled by Guardian Australia shows the growth rate in new cases is now approximately 10% using a five-day average, compared with a high point of 27% on 22 March.

Experts caution that this slowdown is likely to be due to a drop in travel-related cases, and local transmission rates need to be carefully monitored and suppressed.

Prof Peter Collignon, an infectious diseases expert at Australian National University, said there were “some grounds for optimism” in the apparent plateauing in the number of cases being reported.

He said there was a lag of between five to 10 days between an action being taken – such as restricting travel – and that having an impact on the number of cases being reported.

“If what you had was uncontrolled spread, then those numbers would be going up and up,” he said. “I will feel more confident in a week’s time, but there is a suggestion it’s trending downwards.”

Collignon said Australia’s number of reported cases was skewed by arrivals from overseas, which were now being severely restricted.

“What we have to keep an eye on is the transmission with no contact,” he said. “Even though they are small numbers [at the moment] if they go up and up, then that’s a worry. That is transmission that is not controlled and that’s the part of the curve I would be most concerned about.”

Prof Allen Cheng, an epidemiologist at Monash University, agreed and said the plateau in case numbers was “very likely” a reflection of “the fall in overseas travellers, now limited to Australians returning home”.

“It is likely that this [plateau] will only be temporary, as it is likely there will be a shift to more local transmission,” he said.